## Summary
The Prometheus exporter previously linked the
[digitalocean/prometheus-client-c](https://github.com/digitalocean/prometheus-client-c)
(`prom`) library, which is **no longer maintained**. This replaces it
with a minimal, self-contained Prometheus client under `src/prometheus/`
that is built straight from coturn's own sources and implements exactly
the slice of the API coturn uses: counters, gauges, a single default
registry, and the text-exposition serializer.
The vendored client is **drop-in API-compatible**, so `prom_server.c` is
unchanged. Histograms, summaries, custom registries and the upstream
`promhttp` handler are intentionally omitted (coturn serves `/metrics`
with its own libmicrohttpd handler).
The exporter is now **always compiled in**; the runtime `--prometheus`
flag still controls whether it starts. **libmicrohttpd** — actively
maintained — is the only remaining external dependency and is now
required by the build.
## Changes
- **`src/prometheus/{prom.h,prom.c}`** — vendored client. Thread-safe
(per-metric mutex on the increment path, read under lock in the bridge),
label-aware, with proper Prometheus value/label escaping and clean
integer formatting for counters. No deps beyond pthread/libc.
- **CMake** — replace `find_package(Prometheus)` with required
`find_package(MicroHTTPD)`; add `cmake/FindMicroHTTPD.cmake`; remove
dead `cmake/FindPrometheus.cmake` and the `WITH_PROMETHEUS` option.
- **`configure` / `Makefile.in`** — drop `-lprom`, compile the vendored
source.
- **Docker** (`debian`, `alpine`) and **CI** (`ubuntu-build-deps`,
`mingw`) — remove the `prometheus-client-c` build/install steps; keep
libmicrohttpd.
- **`docs/Prometheus.md`** rewritten.
- **`tests/test_prometheus.c`** — Unity tests for counter
accumulation/monotonicity, gauge inc/dec, and label-value escaping.
## Validation
- Full build green; all unit tests pass (incl. new `test_prometheus`);
`clang-format-15` clean.
- Live exporter scraped over HTTP — all `run_tests_prom.sh` scenarios
(default / custom address / port / path, plus the negative case) pass.
---------
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Pavel Punsky
1ec81eff10