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)

---------

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]>

act/runner/job_executor.go

@@ -281,8 +281,8 @@ func newJobExecutor(info jobInfo, sf stepFactory, rc *RunContext) common.Executo
 // finished main-pipeline context. Cleanup has to run even when the run was interrupted, so the
 // returned context always carries a fresh bounded deadline and is never itself cancelled.
 //
-//   - context.Canceled (server cancel): detach from the cancelled context via a fresh root so
-//     the post steps can run.
+//   - context.Canceled (server cancel): detach with WithoutCancel under a fresh job-error
+//     container, so a failing post step cannot turn the cancellation into a failure.
 //   - context.DeadlineExceeded (job timeout): detach the deadline with WithoutCancel, which
 //     keeps the original values — including the job-error container — so the timeout failure and
 //     any post-step error are preserved and the job is still reported as failed.
@@ -290,10 +290,7 @@ func newJobExecutor(info jobInfo, sf stepFactory, rc *RunContext) common.Executo
 func postStepsContext(ctx context.Context) (context.Context, context.CancelFunc) {
 	switch ctx.Err() {
 	case context.Canceled:
-		// The cancelled context is abandoned for a fresh root, which drops the job-error
-		// container installed at the job root. Re-attach a fresh one so a failing post step
-		// records its error via SetJobError instead of panicking on a nil container.
-		return context.WithTimeout(common.WithJobErrorContainer(common.WithLogger(context.Background(), common.Logger(ctx))), 5*time.Minute)
+		return context.WithTimeout(common.WithJobErrorContainer(context.WithoutCancel(ctx)), 5*time.Minute)
 	case context.DeadlineExceeded:
 		return context.WithTimeout(context.WithoutCancel(ctx), 5*time.Minute)
 	default: