Prometheus /metrics Endpoint — counters, histograms, gauges

2026-03-23

observability

What We Built

Prometheus-compatible /metrics endpoint with in-memory collector. No external dependencies.

6 counters: br_requests_total, br_tokens_total, br_cost_usd_total, br_cache_hits_total, br_cache_misses_total, br_guardrail_blocks_total
2 histograms: br_request_duration_seconds, br_ttft_seconds (buckets: 0.1-30s)
4 gauges: br_active_agents, br_circuit_breakers_open, br_cache_hit_rate, br_active_connections

Lockstep Checklist

[x] API: GET /metrics (no auth, Prometheus text format)
[x] Tests: 12 tests for counters, histograms, gauges, default metrics, format
[x] Docs: Ship log