feat: OpenTelemetry tracing (#1206)

Adds OpenTelemetry tracing, configured through the standard `OTEL_*` environment variables and off until an OTLP endpoint is set.

![image.png](https://gitea.com/attachments/c4b49cdf-a32d-403c-94e9-30cd06ee6477)

Each job becomes a trace with a span per step, so operators can see where CI time goes in their existing observability backend. Jobs receive only `TRACEPARENT`, so instrumented tools inside them join the trace without seeing the collector's credentials. Metrics stay on the Prometheus endpoint, which a collector can scrape. See [docs/telemetry.md](https://gitea.com/wmTJc9IK0Q/runner/src/branch/otel/docs/telemetry.md) for setup and security notes.

Also keeps `GITHUB_SHA` and `GITHUB_REF` fixed for a whole run when HEAD moves during `exec` runs and the act tests.

[image.png](https://gitea.com/attachments/c4b49cdf-a32d-403c-94e9-30cd06ee6477)

[image.png](https://gitea.com/attachments/c4b49cdf-a32d-403c-94e9-30cd06ee6477)

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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/1206
Reviewed-by: bircni <[email protected]>
Reviewed-by: silverwind <[email protected]>
Co-authored-by: wmTJc9IK0Q <[email protected]>

README.md

@@ -384,6 +384,10 @@ Both hooks are synchronous and block the job while they run. Either one exiting
 
 See **[docs/job-hooks.md](docs/job-hooks.md)** for the execution order, environment, and platform notes.
 
+#### 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)**.
+
 #### Local job logs (`log.job.dir`)
 
 Set `log.job.dir` to a path and the runner writes a copy of every task's log there as `<start time>-task-<id>.log`: the rows exactly as Gitea received them, with the same secrets masked and the job's result on the last line. Off by default, and what Gitea shows does not change.