[WIP] Fix Go Logger Enhancement workflow failure (#7485)

- [x] Add debug logging to `internal/tracing/fanout.go` using the
existing `logTracing` package-level logger
- [x] Validate changes with `make agent-finished`

Author: lpcox

internal/tracing/fanout.go

@@ -20,14 +20,17 @@ type fanoutExporter struct {
 // When only one exporter is provided it is returned directly to avoid overhead.
 func newFanoutExporter(exporters []sdktrace.SpanExporter) sdktrace.SpanExporter {
 	if len(exporters) == 1 {
+		logTracing.Printf("newFanoutExporter: single exporter, bypassing fanout")
 		return exporters[0]
 	}
+	logTracing.Printf("newFanoutExporter: creating fanout exporter with %d backends", len(exporters))
 	return &fanoutExporter{exporters: exporters}
 }
 
 // forEachExporter calls fn on each underlying exporter concurrently,
-// collecting and joining all errors.
-func (f *fanoutExporter) forEachExporter(fn func(sdktrace.SpanExporter) error) error {
+// collecting and joining all errors. The op label is used in debug log
+// messages to identify which operation is running.
+func (f *fanoutExporter) forEachExporter(op string, fn func(sdktrace.SpanExporter) error) error {
 	var (
 		wg   sync.WaitGroup
 		mu   sync.Mutex
@@ -38,13 +41,17 @@ func (f *fanoutExporter) forEachExporter(fn func(sdktrace.SpanExporter) error) e
 		go func(e sdktrace.SpanExporter) {
 			defer wg.Done()
 			if err := fn(e); err != nil {
+				logTracing.Printf("fanoutExporter.%s: backend (%T) error: %v", op, e, err)
 				mu.Lock()
 				errs = append(errs, err)
 				mu.Unlock()
 			}
 		}(exp)
 	}
 	wg.Wait()
+	if len(errs) > 0 {
+		logTracing.Printf("fanoutExporter.%s: %d/%d backends failed", op, len(errs), len(f.exporters))
+	}
 	return errors.Join(errs...)
 }
 
@@ -53,15 +60,19 @@ func (f *fanoutExporter) forEachExporter(fn func(sdktrace.SpanExporter) error) e
 // delay delivery to the others. Errors from all exporters are collected and
 // joined before returning.
 func (f *fanoutExporter) ExportSpans(ctx context.Context, spans []sdktrace.ReadOnlySpan) error {
-	return f.forEachExporter(func(e sdktrace.SpanExporter) error {
+	logTracing.Printf("fanoutExporter.ExportSpans: exporting %d spans to %d backends", len(spans), len(f.exporters))
+	return f.forEachExporter("ExportSpans", func(e sdktrace.SpanExporter) error {
 		return e.ExportSpans(ctx, spans)
 	})
 }
 
 // Shutdown shuts down each underlying exporter concurrently, collecting any
 // errors. All errors are joined and returned.
 func (f *fanoutExporter) Shutdown(ctx context.Context) error {
-	return f.forEachExporter(func(e sdktrace.SpanExporter) error {
+	logTracing.Printf("fanoutExporter.Shutdown: shutting down %d backends", len(f.exporters))
+	err := f.forEachExporter("Shutdown", func(e sdktrace.SpanExporter) error {
 		return e.Shutdown(ctx)
 	})
+	logTracing.Printf("fanoutExporter.Shutdown: completed")
+	return err
 }

internal/tracing/fanout_test.go

@@ -53,7 +53,7 @@ func TestFanoutExporter_forEachExporter_CallsAllAndJoinsErrors(t *testing.T) {
 	wantErr := errors.New("callback failed")
 	var calls atomic.Int32
 
-	err := exp.forEachExporter(func(sdktrace.SpanExporter) error {
+	err := exp.forEachExporter("test", func(sdktrace.SpanExporter) error {
 		call := calls.Add(1)
 		if call%2 == 0 {
 			return wantErr