contrib/log/slog: avoid logging 0 value trace/span IDs

If the tracer is started but explicitly deactivated, there will be a
span on the context, but the span ID and trace ID will be set to 0.
Adding this information to log outputs seems like unintentional noise
(although it's certainly possible there's a use case I'm not aware of),
so I figured disabling it would be preferred.

The change to explicitly add `tracer.WithTraceEnabled(true)` was so that
the test would be resilient to folks who might have `DD_TRACE_ENABLED`
set explicitly, which is true on my machine.

contrib/log/slog/slog.go

@@ -61,7 +61,7 @@ func (h *handler) Handle(ctx context.Context, rec slog.Record) error {
 	// In case the user has created group loggers, we ignore those and
 	// set them at the root level.
 	span, ok := tracer.SpanFromContext(ctx)
-	if ok {
+	if ok && span.Context().TraceID() != 0 {
 		traceID := strconv.FormatUint(span.Context().TraceID(), 10)
 		spanID := strconv.FormatUint(span.Context().SpanID(), 10)
 

contrib/log/slog/slog_test.go

@@ -47,8 +47,29 @@ func assertLogEntry(t *testing.T, rawEntry, wantMsg, wantLevel string, span trac
 	}
 }
 
+func assertLogEntryNoTrace(t *testing.T, rawEntry, wantMsg, wantLevel string) {
+	t.Helper()
+
+	t.Log(rawEntry)
+
+	var entry map[string]interface{}
+	err := json.Unmarshal([]byte(rawEntry), &entry)
+	require.NoError(t, err)
+	require.NotEmpty(t, entry)
+
+	assert.Equal(t, wantMsg, entry["msg"])
+	assert.Equal(t, wantLevel, entry["level"])
+	assert.NotEmpty(t, entry["time"])
+
+	assert.NotContains(t, entry, ext.LogKeyTraceID)
+	assert.NotContains(t, entry, ext.LogKeySpanID)
+}
+
 func testLogger(t *testing.T, createLogger func(b io.Writer) *slog.Logger, assertExtra func(t *testing.T, entry map[string]interface{})) {
-	tracer.Start(tracer.WithLogger(internallog.DiscardLogger{}))
+	tracer.Start(
+		tracer.WithTraceEnabled(true),
+		tracer.WithLogger(internallog.DiscardLogger{}),
+	)
 	defer tracer.Stop()
 
 	// create the application logger
@@ -73,24 +94,49 @@ func testLogger(t *testing.T, createLogger func(b io.Writer) *slog.Logger, asser
 	assertLogEntry(t, logs[1], "this is an error log with tracing information", "ERROR", span, assertExtra)
 }
 
-func TestNewJSONHandler(t *testing.T) {
-	testLogger(
-		t,
-		func(w io.Writer) *slog.Logger {
-			return slog.New(NewJSONHandler(w, nil))
-		},
-		nil,
+func testLoggerNoTrace(t *testing.T, createLogger func(b io.Writer) *slog.Logger) {
+	tracer.Start(
+		tracer.WithTraceEnabled(false),
+		tracer.WithLogger(internallog.DiscardLogger{}),
 	)
+	defer tracer.Stop()
+
+	// create the application logger
+	var b bytes.Buffer
+	logger := createLogger(&b)
+
+	// start a new span
+	span, ctx := tracer.StartSpanFromContext(context.Background(), "test")
+	defer span.Finish()
+
+	// log a message using the context containing span information
+	logger.Log(ctx, slog.LevelInfo, "this is an info log with tracing information")
+	logger.Log(ctx, slog.LevelError, "this is an error log with tracing information")
+
+	logs := strings.Split(
+		strings.TrimRight(b.String(), "\n"),
+		"\n",
+	)
+	// assert log entries contain trace information
+	require.Len(t, logs, 2)
+	assertLogEntryNoTrace(t, logs[0], "this is an info log with tracing information", "INFO")
+	assertLogEntryNoTrace(t, logs[1], "this is an error log with tracing information", "ERROR")
+}
+
+func TestNewJSONHandler(t *testing.T) {
+	createLogger := func(w io.Writer) *slog.Logger {
+		return slog.New(NewJSONHandler(w, nil))
+	}
+	testLogger(t, createLogger, nil)
+	testLoggerNoTrace(t, createLogger)
 }
 
 func TestWrapHandler(t *testing.T) {
-	testLogger(
-		t,
-		func(w io.Writer) *slog.Logger {
-			return slog.New(WrapHandler(slog.NewJSONHandler(w, nil)))
-		},
-		nil,
-	)
+	createLogger := func(w io.Writer) *slog.Logger {
+		return slog.New(WrapHandler(slog.NewJSONHandler(w, nil)))
+	}
+	testLogger(t, createLogger, nil)
+	testLoggerNoTrace(t, createLogger)
 }
 
 func TestHandlerWithAttrs(t *testing.T) {