fix(spanlogger): set span error tag when log level is error

CHANGELOG.md

@@ -73,6 +73,7 @@
 * [ENHANCEMENT] Distributor: Deduplicate metric metadata when converting PRW 2.0 requests. PRW 2.0 attaches metadata to every series, so a metric family was previously expanded into one `MetricMetadata` per series. #7760
 * [ENHANCEMENT] Querier: Use non-pointer HistogramBucket slice in response codec. #7809
 * [ENHANCEMENT] Update build image and Go version to 1.27.0. #7814
+* [BUGFIX] SpanLogger: Set span error tag when `SpanLogger.Log()` is called with `level=error`, so errors logged through SpanLogger are visible as errors in tracing UIs. #3016
 * [BUGFIX] Querier: Fix queryWithRetry and labelsWithRetry returning (nil, nil) on cancelled context by propagating ctx.Err(). #7370
 * [BUGFIX] Metrics Helper: Fix non-deterministic bucket order in merged histograms by sorting buckets after map iteration, matching Prometheus client library behavior. #7380
 * [BUGFIX] Distributor: Return HTTP 401 Unauthorized when tenant ID resolution fails in the Prometheus Remote Write 2.0 path. #7389

pkg/util/spanlogger/spanlogger.go

@@ -90,6 +90,15 @@ func (s *SpanLogger) Log(kvps ...any) error {
 		return err
 	}
 	s.LogFields(fields...)
+	// Mirror level=error onto the span's error tag so tracing UIs surface it.
+	for i := 0; i+1 < len(kvps); i += 2 {
+		if k, ok := kvps[i].(string); ok && k == "level" {
+			if v, ok := kvps[i+1].(string); ok && v == "error" {
+				ext.Error.Set(s.Span, true)
+			}
+			break
+		}
+	}
 	return nil
 }
 

pkg/util/spanlogger/spanlogger_test.go

@@ -60,6 +60,22 @@ func TestSpanCreatedWithoutTenantTag(t *testing.T) {
 	require.False(t, exist)
 }
 
+func TestSpanLogger_Log_SetsErrorTagOnErrorLevel(t *testing.T) {
+	mockTracer := mocktracer.New()
+	opentracing.SetGlobalTracer(mockTracer)
+
+	logger, _ := New(context.Background(), "test")
+	mockSpan := logger.Span.(*mocktracer.MockSpan)
+
+	// Non-error level should NOT set the error tag.
+	_ = logger.Log("level", "info", "msg", "hello")
+	require.Nil(t, mockSpan.Tag("error"))
+
+	// level=error SHOULD set the span error tag.
+	_ = logger.Log("level", "error", "msg", "something failed")
+	require.Equal(t, true, mockSpan.Tag("error"))
+}
+
 func createSpan(ctx context.Context) *mocktracer.MockSpan {
 	mockTracer := mocktracer.New()
 	opentracing.SetGlobalTracer(mockTracer)