Replace existing traceparent header values (#69)

Co-authored-by: Rafal Augustyniak <rafal.augustyniak@airbnb.com>

Author: Augustyniak

Sources/NautilusTelemetry/Tracing/Span+URLSession.swift

@@ -25,7 +25,7 @@ extension Span {
 		return "\(method) \(target)"
 	}
 
-	/// Add `traceparent` header to a URLRequest if we're sampling
+	/// Sets the `traceparent` header on a URLRequest if we're sampling.
 	/// - Parameter isSampling: whether we are sampling, defaults to InstrumentationSystem.tracer.isSampling
 	/// - Parameter urlRequest: urlRequest to modify
 	public func addTraceHeadersIfSampling(
@@ -34,19 +34,19 @@ extension Span {
 	) {
 		if isSampling {
 			let value = traceParentHeaderValue(sampled: true)
-			urlRequest.addValue(value.1, forHTTPHeaderField: value.0)
+			urlRequest.setValue(value.1, forHTTPHeaderField: value.0)
 		}
 	}
 
-	/// Add `traceparent` header to a URLRequest regardless of sampling state
+	/// Sets the `traceparent` header on a URLRequest regardless of sampling state.
 	/// Sampled flag determined from InstrumentationSystem.tracer.isSampling
 	/// - Parameter urlRequest: urlRequest to modify
 	public func addTraceHeadersUnconditionally(
 		_ urlRequest: inout URLRequest,
 		isSampling: Bool = InstrumentationSystem.tracer.isSampling
 	) {
 		let value = traceParentHeaderValue(sampled: isSampling)
-		urlRequest.addValue(value.1, forHTTPHeaderField: value.0)
+		urlRequest.setValue(value.1, forHTTPHeaderField: value.0)
 	}
 
 	/// Annotates the span with attributes from URLSessionTaskMetrics.

Tests/NautilusTelemetryTests/Tracing/SpanTests.swift

@@ -140,6 +140,28 @@ struct SpanTests {
 		}
 	}
 
+	@Test
+	func traceparentHeaderReplacesExistingValue() throws {
+		let url = try TestUtils.makeURL("https://api.example.com/")
+		let span = tracer.startSpan(name: "test")
+
+		var sampledRequest = URLRequest(url: url)
+		sampledRequest.setValue("existing", forHTTPHeaderField: "traceparent")
+		span.addTraceHeadersIfSampling(&sampledRequest, isSampling: true)
+		#expect(
+			sampledRequest.value(forHTTPHeaderField: "traceparent")
+				== span.traceParentHeaderValue(sampled: true).1
+		)
+
+		var unsampledRequest = URLRequest(url: url)
+		unsampledRequest.setValue("existing", forHTTPHeaderField: "traceparent")
+		span.addTraceHeadersUnconditionally(&unsampledRequest, isSampling: false)
+		#expect(
+			unsampledRequest.value(forHTTPHeaderField: "traceparent")
+				== span.traceParentHeaderValue(sampled: false).1
+		)
+	}
+
 	@Test
 	func throwingSpan() throws {
 		#expect(throws: TestError.self) {