prom-client
Prometheus client library for Node.js. prom-client provides Counter, Gauge, Histogram, Summary metric types and a /metrics HTTP endpoint that Prometheus scrapes. Includes default process metrics (CPU, memory, event loop lag, GC) out of the box. The standard way to instrument Node.js agent applications for Prometheus/Grafana observability.
Score Breakdown
⚙ Agent Friendliness
🔒 Security
The /metrics endpoint exposes operational data — restrict access to Prometheus server IPs. Avoid exposing sensitive data (user IDs, PII) in metric labels. No auth built-in — implement at reverse proxy level.
⚡ Reliability
Best When
Your infrastructure uses Prometheus for metrics collection and Grafana for visualization — prom-client is the standard Node.js instrumentation library.
Avoid When
Your observability stack uses Datadog, New Relic, or another non-Prometheus system — use their native Node.js clients instead.
Use Cases
- • Instrument Node.js agent services with request duration histograms, error counters, and queue depth gauges for Prometheus scraping
- • Track agent LLM API call metrics — request count, latency distribution, token usage, error rates — visible in Grafana dashboards
- • Monitor agent resource consumption (heap, event loop lag, GC pause) using prom-client's built-in default process metrics
- • Expose agent business metrics (tasks completed, documents processed, active sessions) as Prometheus metrics for operational visibility
- • Set up alerting rules based on prom-client exported metrics — trigger PagerDuty when agent error rate exceeds threshold
Not For
- • Push-based metrics systems (Datadog, StatsD) — prom-client is Prometheus pull-based; use hot-shots or node-statsd for StatsD/Datadog
- • Log-based metrics — prom-client is for numerical metrics, not log events; use Winston/Pino alongside prom-client
- • Distributed tracing — use OpenTelemetry for trace-level observability; prom-client handles metrics only
Interface
Authentication
No auth in prom-client itself — secure the /metrics endpoint at the HTTP server/proxy level. Prometheus server needs network access to scrape the endpoint.
Pricing
prom-client is open source and free. Prometheus server infrastructure costs are separate.
Agent Metadata
Known Gotchas
- ⚠ Metric names must be globally unique in the default Prometheus registry — duplicate registration across modules throws; use a single module to define all metrics
- ⚠ High-cardinality label values (userId, requestId, URL) cause Prometheus memory explosion — only use bounded cardinality labels (status_code, method, endpoint pattern)
- ⚠ Default metrics (collectDefaultMetrics()) register process-level metrics — call once per process, not per request; calling multiple times registers duplicate metrics and throws
- ⚠ Histogram buckets must be configured upfront and sorted — use linearBuckets() or exponentialBuckets() helpers; wrong bucket ranges miss latency tail percentiles
- ⚠ prom-client doesn't push metrics — Prometheus pulls from /metrics on a scrape interval; agents must keep the /metrics endpoint running and accessible to the Prometheus server
- ⚠ Multi-process Node.js (cluster mode) requires prom-client's AggregatorRegistry for aggregating metrics across workers — single-process default registry doesn't aggregate cluster worker metrics
Alternatives
Full Evaluation Report
Detailed scoring breakdown, competitive positioning, security analysis, and improvement recommendations for prom-client.
Scores are editorial opinions as of 2026-03-06.