Metrics Collection

Prometheus-compatible metrics for visibility into system behavior.

When to Use This Skill

• Need visibility into request rates and latencies
• Want to track business metrics (signups, conversions)
• Building dashboards and alerts
• Debugging performance issues

Core Concepts

Three metric types cover most use cases:

Type	Use Case	Example
Counter	Things that only go up	Requests, errors, events
Gauge	Current value	Active connections, queue size
Histogram	Distribution of values	Request latency, response sizes

Implementation

TypeScript

interface CounterMetric {
  name: string;
  help: string;
  labels: string[];
  values: Map<string, number>;
}

interface GaugeMetric {
  name: string;
  help: string;
  labels: string[];
  values: Map<string, number>;
}

interface HistogramMetric {
  name: string;
  help: string;
  labels: string[];
  buckets: number[];
  values: Map<string, { count: number; sum: number; buckets: number[] }>;
}

class MetricsRegistry {
  private counters = new Map<string, CounterMetric>();
  private gauges = new Map<string, GaugeMetric>();
  private histograms = new Map<string, HistogramMetric>();

  // Counter methods
  registerCounter(name: string, help: string, labels: string[] = []): void {
    if (!this.counters.has(name)) {
      this.counters.set(name, { name, help, labels, values: new Map() });
    }
  }

  incrementCounter(name: string, labels: Record<string, string> = {}, value = 1): void {
    const counter = this.counters.get(name);
    if (!counter) return;

    const key = this.labelsToKey(labels);
    const current = counter.values.get(key) || 0;
    counter.values.set(key, current + value);
  }

  // Gauge methods
  registerGauge(name: string, help: string, labels: string[] = []): void {
    if (!this.gauges.has(name)) {
      this.gauges.set(name, { name, help, labels, values: new Map() });
    }
  }

  setGauge(name: string, value: number, labels: Record<string, string> = {}): void {
    const gauge = this.gauges.get(name);
    if (!gauge) return;

    const key = this.labelsToKey(labels);
    gauge.values.set(key, value);
  }

  incrementGauge(name: string, labels: Record<string, string> = {}, value = 1): void {
    const gauge = this.gauges.get(name);
    if (!gauge) return;

    const key = this.labelsToKey(labels);
    const current = gauge.values.get(key) || 0;
    gauge.values.set(key, current + value);
  }

  decrementGauge(name: string, labels: Record<string, string> = {}, value = 1): void {
    this.incrementGauge(name, labels, -value);
  }

  // Histogram methods
  registerHistogram(
    name: string,
    help: string,
    labels: string[] = [],
    buckets: number[] = [0.005, 0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1, 2.5, 5, 10]
  ): void {
    if (!this.histograms.has(name)) {
      this.histograms.set(name, { name, help, labels, buckets, values: new Map() });
    }
  }

  observeHistogram(name: string, value: number, labels: Record<string, string> = {}): void {
    const histogram = this.histograms.get(name);
    if (!histogram) return;

    const key = this.labelsToKey(labels);
    let data = histogram.values.get(key);

    if (!data) {
      data = { count: 0, sum: 0, buckets: new Array(histogram.buckets.length).fill(0) };
      histogram.values.set(key, data);
    }

    data.count++;
    data.sum += value;

    for (let i = 0; i < histogram.buckets.length; i++) {
      if (value <= histogram.buckets[i]) {
        data.buckets[i]++;
      }
    }
  }

  // Timer helper
  startTimer(histogramName: string, labels: Record<string, string> = {}): () => void {
    const start = performance.now();
    return () => {
      const duration = (performance.now() - start) / 1000;
      this.observeHistogram(histogramName, duration, labels);
    };
  }

  // Export to Prometheus format
  toPrometheus(): string {
    const lines: string[] = [];

    for (const counter of this.counters.values()) {
      lines.push(`# HELP ${counter.name} ${counter.help}`);
      lines.push(`# TYPE ${counter.name} counter`);
      for (const [labels, value] of counter.values) {
        const labelStr = labels ? `{${labels}}` : '';
        lines.push(`${counter.name}${labelStr} ${value}`);
      }
    }

    for (const gauge of this.gauges.values()) {
      lines.push(`# HELP ${gauge.name} ${gauge.help}`);
      lines.push(`# TYPE ${gauge.name} gauge`);
      for (const [labels, value] of gauge.values) {
        const labelStr = labels ? `{${labels}}` : '';
        lines.push(`${gauge.name}${labelStr} ${value}`);
      }
    }

    for (const histogram of this.histograms.values()) {
      lines.push(`# HELP ${histogram.name} ${histogram.help}`);
      lines.push(`# TYPE ${histogram.name} histogram`);
      for (const [labels, data] of histogram.values) {
        const labelStr = labels ? `${labels},` : '';
        for (let i = 0; i < histogram.buckets.length; i++) {
          lines.push(`${histogram.name}_bucket{${labelStr}le="${histogram.buckets[i]}"} ${data.buckets[i]}`);
        }
        lines.push(`${histogram.name}_bucket{${labelStr}le="+Inf"} ${data.count}`);
        lines.push(`${histogram.name}_sum{${labels}} ${data.sum}`);
        lines.push(`${histogram.name}_count{${labels}} ${data.count}`);
      }
    }

    return lines.join('\n');
  }

  toJSON(): object {
    return {
      counters: Object.fromEntries(
        Array.from(this.counters.entries()).map(([name, metric]) => [
          name, Object.fromEntries(metric.values),
        ])
      ),
      gauges: Object.fromEntries(
        Array.from(this.gauges.entries()).map(([name, metric]) => [
          name, Object.fromEntries(metric.values),
        ])
      ),
      histograms: Object.fromEntries(
        Array.from(this.histograms.entries()).map(([name, metric]) => [
          name, Object.fromEntries(metric.values),
        ])
      ),
    };
  }

  private labelsToKey(labels: Record<string, string>): string {
    return Object.entries(labels)
      .sort(([a], [b]) => a.localeCompare(b))
      .map(([k, v]) => `${k}="${v}"`)
      .join(',');
  }
}

export const metrics = new MetricsRegistry();

// Pre-register common metrics
metrics.registerCounter('http_requests_total', 'Total HTTP requests', ['method', 'path', 'status']);
metrics.registerCounter('errors_total', 'Total errors', ['type', 'source']);
metrics.registerHistogram('http_request_duration_seconds', 'HTTP request duration', ['method', 'path']);
metrics.registerGauge('active_connections', 'Active connections');
metrics.registerGauge('queue_size', 'Queue size', ['queue']);

Python

from dataclasses import dataclass, field
from typing import Dict, List, Optional, Callable
import time


@dataclass
class CounterMetric:
    name: str
    help: str
    labels: List[str]
    values: Dict[str, float] = field(default_factory=dict)


@dataclass
class GaugeMetric:
    name: str
    help: str
    labels: List[str]
    values: Dict[str, float] = field(default_factory=dict)


@dataclass
class HistogramData:
    count: int = 0
    sum: float = 0
    buckets: List[int] = field(default_factory=list)


@dataclass
class HistogramMetric:
    name: str
    help: str
    labels: List[str]
    bucket_bounds: List[float]
    values: Dict[str, HistogramData] = field(default_factory=dict)


class MetricsRegistry:
    DEFAULT_BUCKETS = [0.005, 0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1, 2.5, 5, 10]
    
    def __init__(self):
        self._counters: Dict[str, CounterMetric] = {}
        self._gauges: Dict[str, GaugeMetric] = {}
        self._histograms: Dict[str, HistogramMetric] = {}
    
    def register_counter(self, name: str, help: str, labels: List[str] = None) -> None:
        if name not in self._counters:
            self._counters[name] = CounterMetric(name, help, labels or [])
    
    def increment_counter(self, name: str, labels: Dict[str, str] = None, value: float = 1) -> None:
        counter = self._counters.get(name)
        if not counter:
            return
        
        key = self._labels_to_key(labels or {})
        counter.values[key] = counter.values.get(key, 0) + value
    
    def register_gauge(self, name: str, help: str, labels: List[str] = None) -> None:
        if name not in self._gauges:
            self._gauges[name] = GaugeMetric(name, help, labels or [])
    
    def set_gauge(self, name: str, value: float, labels: Dict[str, str] = None) -> None:
        gauge = self._gauges.get(name)
        if not gauge:
            return
        
        key = self._labels_to_key(labels or {})
        gauge.values[key] = value
    
    def increment_gauge(self, name: str, labels: Dict[str, str] = None, value: float = 1) -> None:
        gauge = self._gauges.get(name)
        if not gauge:
            return
        
        key = self._labels_to_key(labels or {})
        gauge.values[key] = gauge.values.get(key, 0) + value
    
    def register_histogram(
        self, name: str, help: str, labels: List[str] = None, buckets: List[float] = None
    ) -> None:
        if name not in self._histograms:
            self._histograms[name] = HistogramMetric(
                name, help, labels or [], buckets or self.DEFAULT_BUCKETS
            )
    
    def observe_histogram(self, name: str, value: float, labels: Dict[str, str] = None) -> None:
        histogram = self._histograms.get(name)
        if not histogram:
            return
        
        key = self._labels_to_key(labels or {})
        if key not in histogram.values:
            histogram.values[key] = HistogramData(
                buckets=[0] * len(histogram.bucket_bounds)
            )
        
        data = histogram.values[key]
        data.count += 1
        data.sum += value
        
        for i, bound in enumerate(histogram.bucket_bounds):
            if value <= bound:
                data.buckets[i] += 1
    
    def start_timer(self, histogram_name: str, labels: Dict[str, str] = None) -> Callable[[], None]:
        start = time.perf_counter()
        
        def end_timer():
            duration = time.perf_counter() - start
            self.observe_histogram(histogram_name, duration, labels)
        
        return end_timer
    
    def to_prometheus(self) -> str:
        lines = []
        
        for counter in self._counters.values():
            lines.append(f"# HELP {counter.name} {counter.help}")
            lines.append(f"# TYPE {counter.name} counter")
            for labels, value in counter.values.items():
                label_str = f"{{{labels}}}" if labels else ""
                lines.append(f"{counter.name}{label_str} {value}")
        
        for gauge in self._gauges.values():
            lines.append(f"# HELP {gauge.name} {gauge.help}")
            lines.append(f"# TYPE {gauge.name} gauge")
            for labels, value in gauge.values.items():
                label_str = f"{{{labels}}}" if labels else ""
                lines.append(f"{gauge.name}{label_str} {value}")
        
        for histogram in self._histograms.values():
            lines.append(f"# HELP {histogram.name} {histogram.help}")
            lines.append(f"# TYPE {histogram.name} histogram")
            for labels, data in histogram.values.items():
                label_prefix = f"{labels}," if labels else ""
                for i, bound in enumerate(histogram.bucket_bounds):
                    lines.append(f'{histogram.name}_bucket{{{label_prefix}le="{bound}"}} {data.buckets[i]}')
                lines.append(f'{histogram.name}_bucket{{{label_prefix}le="+Inf"}} {data.count}')
                lines.append(f"{histogram.name}_sum{{{labels}}} {data.sum}")
                lines.append(f"{histogram.name}_count{{{labels}}} {data.count}")
        
        return "\n".join(lines)
    
    def _labels_to_key(self, labels: Dict[str, str]) -> str:
        return ",".join(f'{k}="{v}"' for k, v in sorted(labels.items()))


# Singleton
metrics = MetricsRegistry()

# Pre-register common metrics
metrics.register_counter("http_requests_total", "Total HTTP requests", ["method", "path", "status"])
metrics.register_counter("errors_total", "Total errors", ["type", "source"])
metrics.register_histogram("http_request_duration_seconds", "HTTP request duration", ["method", "path"])
metrics.register_gauge("active_connections", "Active connections")
metrics.register_gauge("queue_size", "Queue size", ["queue"])

Usage Examples

HTTP Request Tracking

async function withMetrics(
  handler: () => Promise<Response>,
  method: string,
  path: string
): Promise<Response> {
  const endTimer = metrics.startTimer('http_request_duration_seconds', { method, path });

  try {
    const response = await handler();
    metrics.incrementCounter('http_requests_total', {
      method, path, status: String(response.status),
    });
    return response;
  } catch (error) {
    metrics.incrementCounter('http_requests_total', { method, path, status: '500' });
    metrics.incrementCounter('errors_total', { type: 'http', source: path });
    throw error;
  } finally {
    endTimer();
  }
}

Queue Monitoring

class JobQueue {
  private queue: Job[] = [];

  add(job: Job): void {
    this.queue.push(job);
    metrics.setGauge('queue_size', this.queue.length, { queue: 'jobs' });
  }

  process(): Job | undefined {
    const job = this.queue.shift();
    metrics.setGauge('queue_size', this.queue.length, { queue: 'jobs' });
    return job;
  }
}

Business Metrics

metrics.registerCounter('predictions_generated', 'Predictions generated', ['tier']);
metrics.registerCounter('user_signups', 'User signups', ['source']);

async function generatePrediction(userId: string, tier: string) {
  const endTimer = metrics.startTimer('prediction_latency_seconds');
  try {
    const prediction = await mlPipeline.generate();
    metrics.incrementCounter('predictions_generated', { tier });
    return prediction;
  } finally {
    endTimer();
  }
}

Metrics Endpoint

app.get('/metrics', (req, res) => {
  res.set('Content-Type', 'text/plain; charset=utf-8');
  res.send(metrics.toPrometheus());
});

Best Practices

1. Use consistent naming (snake_case, units in name)
2. Keep cardinality low (avoid high-cardinality labels)
3. Pre-register metrics at startup
4. Use histograms for latencies, not gauges
5. Include units in metric names (_seconds, _bytes)

Common Mistakes

• High cardinality labels (user_id as label)
• Using gauges for latency (use histograms)
• Not pre-registering metrics
• Missing units in names
• Too many buckets in histograms

Related Patterns

• health-checks - Health endpoints for probes
• anomaly-detection - Alert on metric anomalies
• logging-observability - Correlate logs with metrics