starlette_exporter

starlette_exporter is a Prometheus exporter implemented as ASGI middleware for Starlette and FastAPI apps. Adding it instruments your application with request-level metrics — total request counts, request-duration histograms, and an in-progress gauge — each labelled by HTTP method, path, and status code.

It ships a PrometheusMiddleware you attach to your app and a handle_metrics handler you mount at /metrics for Prometheus to scrape. Options cover metric prefixes, default labels, path templating for route parameters, exemplars, custom metrics, and multiprocess (gunicorn) deployments.

What You Get

• PrometheusMiddleware that records request totals, durations, and in-progress counts
• A handle_metrics handler to expose a Prometheus-scrapeable /metrics endpoint
• Automatic labels for HTTP method, path, and status code
• Configurable app name, metric prefix, default labels, and path templating for route params
• Support for exemplars, custom metrics, and multiprocess (gunicorn) deployments

Common Use Cases

• Instrumenting a FastAPI service with Prometheus metrics for latency and error-rate dashboards
• Exposing RED metrics (rate, errors, duration) from a Starlette ASGI app
• Adding per-route request histograms to power Grafana SLO panels
• Collecting metrics across multiple gunicorn workers in multiprocess mode

Under The Hood

Architecture - The core is an ASGI middleware that wraps each request: it increments an in-progress gauge, times the request, and on completion records a counter and a duration histogram with method/path/status_code labels, using route templating so path parameters collapse to a stable label (e.g. /items/{id}). The handle_metrics route renders the prometheus_client registry, honoring multiprocess mode when configured. Tech Stack - Pure Python built on Starlette’s middleware interface and the official prometheus_client library; works unchanged under FastAPI since FastAPI is built on Starlette. Packaged with a modern pyproject/poetry setup. Code Quality - A focused, mature codebase with a test suite covering middleware behavior, options, and multiprocess handling; development activity is low, consistent with a small, feature-complete utility. API Design - Integration is two lines — add the middleware, add the route — and every behavior (prefix, labels, filtering unhandled paths, exemplars, custom metrics) is a keyword option, keeping the common case trivial while leaving room for advanced tuning.