enhance: export the Prometheus metrics over OpenTelemetry (#1248)

Push the runner's Prometheus metrics to the OTLP endpoint next to the job traces, so both land in the same backend.

The `/metrics` registry is converted as the [Prometheus compatibility spec](https://opentelemetry.io/docs/specs/otel/compatibility/prometheus_and_openmetrics/) describes and sent through the existing gRPC-free client, so names stay unchanged and no second metrics stack is linked. Metrics are on by default and pushed every interval and once on shutdown, so ephemeral runners report too.

New standard variables, each `OTEL_EXPORTER_OTLP_*` one with `TRACES` and `METRICS` variants:

- `OTEL_METRICS_EXPORTER`, `none` turns metrics off
- `OTEL_METRIC_EXPORT_INTERVAL`, `OTEL_METRIC_EXPORT_TIMEOUT`
- `OTEL_EXPORTER_OTLP_TIMEOUT`, `OTEL_EXPORTER_OTLP_COMPRESSION`
- `OTEL_EXPORTER_OTLP_CERTIFICATE`, `OTEL_EXPORTER_OTLP_CLIENT_CERTIFICATE`, `OTEL_EXPORTER_OTLP_CLIENT_KEY`

---------

Co-authored-by: silverwind <[email protected]>
Co-authored-by: silverwind <[email protected]>
Co-authored-by: bircni <[email protected]>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1248
Reviewed-by: silverwind <[email protected]>
Reviewed-by: bircni <[email protected]>
Co-authored-by: wmTJc9IK0Q <[email protected]>

README.md

@@ -382,7 +382,7 @@ See **[docs/job-hooks.md](docs/job-hooks.md)** for the execution order, environm
 
 #### OpenTelemetry (`OTEL_*`)
 
-Set `OTEL_EXPORTER_OTLP_ENDPOINT` in the runner's environment to send a trace of every job to an OpenTelemetry collector. See **[docs/telemetry.md](docs/telemetry.md)**.
+Set `OTEL_EXPORTER_OTLP_ENDPOINT` in the runner's environment to send a trace of every job, and the runner's metrics, to an OpenTelemetry collector. See **[docs/telemetry.md](docs/telemetry.md)**.
 
 #### Local job logs (`log.job.dir`)