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/ghcontext/github_context_test.go

@@ -214,4 +214,17 @@ func TestSetSha(t *testing.T) {
 			assert.Equal(t, table.sha, ghc.Sha)
 		})
 	}
+
+	t.Run("pinned checkout keeps the first sha and ref", func(t *testing.T) {
+		ctx := WithPinnedCheckout(context.Background())
+		for _, head := range []string{"first", "moved"} {
+			findGitRevision = func(context.Context, string) (string, string, error) { return "", head, nil }
+			findGitRef = func(context.Context, string) (string, error) { return "refs/heads/" + head, nil }
+			ghc := &model.GithubContext{Event: map[string]any{}}
+			SetSha(ctx, ghc, "/some/dir")
+			SetRef(ctx, ghc, "/some/dir")
+			assert.Equal(t, "first", ghc.Sha)
+			assert.Equal(t, "refs/heads/first", ghc.Ref)
+		}
+	})
 }