Feat: DO-11819 - add HTTP request metrics middleware

[momustafa-causalens]

Motivation and Context

Dara already exposes Prometheus metrics on port 10000 (task runtimes, DV runtimes, cache size), but has no HTTP request-level metrics. Adding http_requests_total and http_request_duration_seconds enables the observability service to surface request rate, error rate, and latency for deployed Dara apps.

Implementation Description

• New metrics/http.py — defines http_requests_total (Counter) and http_request_duration_seconds (Histogram) with labels method, status, path. A PrometheusMiddleware (Starlette BaseHTTPMiddleware) wraps every request and records both metrics. Uses the matched route pattern (e.g. /api/components/{uid}) for the path label to avoid unbounded cardinality.

• Middleware registration in main.py — gated by the same conditions as the existing metrics server (DARA_DISABLE_METRICS != TRUE and DARA_TEST_FLAG not set), so tests are unaffected.

• Exports added to metrics/__init__.py — HTTP_REQUESTS_TOTAL and HTTP_REQUEST_DURATION_SECONDS are available for downstream code if needed.

No changes to the existing /status endpoint, metrics server startup, or prometheus_client dependency — all already in place.

Note

Not sure if we need a changelog update for this sort of change

Any new dependencies Introduced

None.

How Has This Been Tested?

• Targeted test suite passes (70 passed). The middleware is not registered during tests (DARA_TEST_FLAG guard), so existing tests are unaffected.
• Import verification: from dara.core.metrics.http import PrometheusMiddleware succeeds.

PR Checklist:

• I have implemented all requirements? (see JIRA, project documentation).
• I am not affecting someone else's work, If I am, they are included as a reviewer.
• I have added relevant tests (unit, integration or regression).
• I have added comments to all the bits that are hard to follow.
• I have added/updated Documentation.
• I have updated the appropriate changelog with a line for my changes.

Screenshots (if appropriate):

N/A — backend-only change. Metrics are visible via curl http://localhost:10000/metrics.

[krzysztof-causalens]

Could we add a brief changelog entry, just say that we extended metrics to include http stats