feat: Add W3C traceparent support to tracing client and utils

CHANGELOG.md

@@ -25,6 +25,8 @@ await SentryFlutter.init(
 ```
 - Make hierarchical exception grouping opt-in ([#2858](https://github.com/getsentry/sentry-dart/pull/2858))
 
+- Outgoing HTTP requests now contain the `traceparent` header to facilitate integration with servers that only support the [W3C Trace Context](https://www.w3.org/TR/trace-context/). ([#2843](https://github.com/getsentry/sentry-dart/pull/2843))
+
 ### Fixes
 
 - Trace propagation in HTTP tracing clients not correctly set up if performance is disabled ([#2850](https://github.com/getsentry/sentry-dart/pull/2850))

dart/lib/src/utils/add_tracing_headers_to_http_request.dart

@@ -7,6 +7,7 @@ void addTracingHeadersToHttpHeader(Map<String, dynamic> headers, Hub hub,
     {ISentrySpan? span}) {
   if (span != null) {
     addSentryTraceHeaderFromSpan(span, headers);
+    addW3CHeaderFromSpan(span, headers);
     addBaggageHeaderFromSpan(
       span,
       headers,
@@ -18,6 +19,7 @@ void addTracingHeadersToHttpHeader(Map<String, dynamic> headers, Hub hub,
 
     final traceHeader = propagationContext.toSentryTrace();
     addSentryTraceHeader(traceHeader, headers);
+    addW3CHeaderFromSentryTrace(traceHeader, headers);
 
     final baggageHeader = propagationContext.toBaggageHeader();
     if (baggageHeader != null) {

dart/lib/src/utils/tracing_utils.dart

@@ -11,6 +11,22 @@ void addSentryTraceHeader(
   headers[traceHeader.name] = traceHeader.value;
 }
 
+void addW3CHeaderFromSpan(ISentrySpan span, Map<String, dynamic> headers) {
+  final traceHeader = span.toSentryTrace();
+  addW3CHeaderFromSentryTrace(traceHeader, headers);
+}
+
+void addW3CHeaderFromSentryTrace(
+    SentryTraceHeader traceHeader, Map<String, dynamic> headers) {
+  headers['traceparent'] = formatAsW3CHeader(traceHeader);
+}
+
+String formatAsW3CHeader(SentryTraceHeader traceHeader) {
+  final sampled = traceHeader.sampled;
+  final traceFlags = sampled != null && sampled ? '01' : '00';
+  return '00-${traceHeader.traceId}-${traceHeader.spanId}-$traceFlags';
+}
+
 void addBaggageHeaderFromSpan(
   ISentrySpan span,
   Map<String, dynamic> headers, {

dart/test/utils/tracing_utils_test.dart

@@ -112,6 +112,41 @@ void main() {
       expect(headers[baggage.name], newValue);
     });
 
+    group('$addW3CHeaderFromSentryTrace', () {
+      final fixture = Fixture();
+      final headerName = 'traceparent';
+      final headerValue =
+          '00-${fixture._context.traceId}-${fixture._context.spanId}-01';
+
+      test('formats SentryTraceHeader as W3C header', () {
+        final sut = fixture.getSut();
+        final sentryHeader = sut.toSentryTrace();
+
+        final w3cHeader = formatAsW3CHeader(sentryHeader);
+
+        expect(w3cHeader, headerValue);
+      });
+
+      test('adds W3C traceparent from span', () {
+        final headers = <String, String>{};
+        final sut = fixture.getSut();
+
+        addW3CHeaderFromSpan(sut, headers);
+
+        expect(headers[headerName], headerValue);
+      });
+
+      test('adds W3C traceparent header', () {
+        final headers = <String, String>{};
+        final sut = fixture.getSut();
+        final sentryHeader = sut.toSentryTrace();
+
+        addW3CHeaderFromSentryTrace(sentryHeader, headers);
+
+        expect(headers[headerName], headerValue);
+      });
+    });
+
     test('overwrites duplicate key values', () {
       final headers = <String, String>{};
       final oldValue =