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]>

internal/app/cmd/daemon.go

@@ -25,6 +25,7 @@ import (
 	"gitea.com/gitea/runner/internal/pkg/lock"
 	"gitea.com/gitea/runner/internal/pkg/metrics"
 	"gitea.com/gitea/runner/internal/pkg/report"
+	"gitea.com/gitea/runner/internal/pkg/telemetry"
 	"gitea.com/gitea/runner/internal/pkg/ver"
 
 	"connectrpc.com/connect"
@@ -71,6 +72,18 @@ func runDaemon(ctx context.Context, daemArgs *daemonArgs, configFile *string) fu
 			defer func() { _ = releaseLock() }()
 		}
 
+		shutdownTelemetry, err := telemetry.Setup(ctx, reg.UUID, reg.Name)
+		if err != nil {
+			log.WithError(err).Warn("OpenTelemetry export failed to start")
+		}
+		defer func() {
+			ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+			defer cancel()
+			if err := shutdownTelemetry(ctx); err != nil {
+				log.WithError(err).Warn("OpenTelemetry export failed to flush")
+			}
+		}()
+
 		lbls := resolveLabels(daemArgs.Labels, cfg.Runner.Labels, reg.Labels)
 
 		ls := labels.Labels{}