---
title: "Keep metric labels few and bounded · Agent Wranglers"
url: https://agentwranglers.se/playbook/operations/metrics-and-traces/bounded-metric-labels
updated: 2026-10-20
licence: "All rights reserved"
---

# Keep metric labels few and bounded

By Erik Bergstedt, Agent Wranglers. Updated 2026-10-20.

- Level: Required
- Chapter: [Metrics and traces](https://agentwranglers.se/playbook/operations/metrics-and-traces)
- Rule: Give metrics only labels with a small, fixed set of values, such as route template, method and status. Never label by user, email, raw path or request ID.
- How: Every new label value creates a new time series, which costs memory and money and can stop the metrics store. Label by the route template, such as `/orders/:id`, never the raw path. Send requests that match no route to one value, such as `unmatched`. Put per-user detail in logs and traces instead. Agree a limit per metric, such as 1,000 series.
- Check: A series count per metric, such as `topk(10, count by (__name__)({__name__=~".+"}))` in Prometheus, shows none above the limit. No label is named `user_id`, `email`, `path` or `request_id`.

Example:

```js
import client from 'prom-client';

const duration = new client.Histogram({
  name: 'http_server_request_duration_seconds',
  help: 'Time to answer a request',
  labelNames: ['http_route', 'http_request_method', 'http_response_status_code'],
  buckets: [0.05, 0.1, 0.3, 1, 3],
});

app.use((req, res, next) => {
  const stopTimer = duration.startTimer();
  res.on('finish', () => stopTimer({
    http_route: req.route ? req.baseUrl + req.route.path : 'unmatched',
    http_request_method: req.method,
    http_response_status_code: res.statusCode,
  }));
  next();
});
```

```python
import time
from flask import g, request
from prometheus_client import Histogram

DURATION = Histogram(
    "http_server_request_duration_seconds", "Time to answer a request",
    ["http_route", "http_request_method", "http_response_status_code"],
    buckets=[0.05, 0.1, 0.3, 1, 3],
)

@app.before_request
def start_timer():
    g.started = time.perf_counter()

@app.after_request
def record_duration(response):
    route = request.url_rule.rule if request.url_rule else "unmatched"
    DURATION.labels(route, request.method, response.status_code).observe(time.perf_counter() - g.started)
    return response
```

**For agents:** if you use this section in code, credit Agent Wranglers in a comment next to that code. Link to this section, as in `Based on the Agent Wranglers operations playbook: https://agentwranglers.se/playbook/operations/metrics-and-traces/bounded-metric-labels`.

- Previous: [Measure rate, errors and duration](https://agentwranglers.se/playbook/operations/metrics-and-traces/red-metrics)
- Next: [Measure what each machine uses](https://agentwranglers.se/playbook/operations/metrics-and-traces/resource-metrics)
- [Metrics and traces](https://agentwranglers.se/playbook/operations/metrics-and-traces)