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.go

@@ -12,6 +12,7 @@ import (
 	"context"
 	"fmt"
 	"strings"
+	"sync"
 
 	"gitea.com/gitea/runner/act/common"
 	"gitea.com/gitea/runner/act/common/git"
@@ -25,6 +26,26 @@ var (
 	findGithubRepo  = git.FindGithubRepo
 )
 
+type pinnedKey struct{}
+
+// WithPinnedCheckout keeps a run's sha and ref fixed when HEAD moves.
+func WithPinnedCheckout(ctx context.Context) context.Context {
+	if ctx.Value(pinnedKey{}) != nil {
+		return ctx
+	}
+	return context.WithValue(ctx, pinnedKey{}, &sync.Map{})
+}
+
+func pinned(ctx context.Context, key string, lookup func() (string, error)) (string, error) {
+	cache, ok := ctx.Value(pinnedKey{}).(*sync.Map)
+	if !ok {
+		return lookup()
+	}
+	cached, _ := cache.LoadOrStore(key, sync.OnceValues(lookup))
+	lookupOnce, _ := cached.(func() (string, error))
+	return lookupOnce()
+}
+
 // SetRef resolves the ref of the context from its event payload, falling back
 // to the ref checked out in repoPath.
 func SetRef(ctx context.Context, ghc *model.GithubContext, repoPath string) {
@@ -51,7 +72,7 @@ func SetRef(ctx context.Context, ghc *model.GithubContext, repoPath string) {
 	}
 
 	if ghc.Ref == "" {
-		ref, err := findGitRef(ctx, repoPath)
+		ref, err := pinned(ctx, "ref:"+repoPath, func() (string, error) { return findGitRef(ctx, repoPath) })
 		if err != nil {
 			logger.Warningf("unable to get git ref: %v", err)
 		} else {
@@ -95,7 +116,10 @@ func SetSha(ctx context.Context, ghc *model.GithubContext, repoPath string) {
 	}
 
 	if ghc.Sha == "" {
-		_, sha, err := findGitRevision(ctx, repoPath)
+		sha, err := pinned(ctx, "sha:"+repoPath, func() (string, error) {
+			_, revision, err := findGitRevision(ctx, repoPath)
+			return revision, err
+		})
 		if err != nil {
 			logger.Warningf("unable to get git revision: %v", err)
 		} else {