[Tracing] Implement trace header context propagation

Somewhere along the way of reviewing previous work this bit got lost;

This implements context propagation by injecting the apropriate headers
from service context into the headers when forming a `Prepared` http
request.

Sources/AsyncHTTPClient/AsyncAwait/HTTPClient+execute.swift

@@ -92,7 +92,12 @@ extension HTTPClient {
 
         // this loop is there to follow potential redirects
         while true {
-            let preparedRequest = try HTTPClientRequest.Prepared(currentRequest, dnsOverride: configuration.dnsOverride)
+            let preparedRequest =
+                try HTTPClientRequest.Prepared(
+                    currentRequest,
+                    tracing: self.configuration.tracing,
+                    dnsOverride: configuration.dnsOverride
+                )
             let response = try await {
                 var response = try await self.executeCancellable(preparedRequest, deadline: deadline, logger: logger)
 

Sources/AsyncHTTPClient/AsyncAwait/HTTPClientRequest+Prepared.swift

@@ -12,9 +12,11 @@
 //
 //===----------------------------------------------------------------------===//
 
+import Instrumentation
 import NIOCore
 import NIOHTTP1
 import NIOSSL
+import ServiceContextModule
 
 import struct Foundation.URL
 
@@ -45,7 +47,11 @@ extension HTTPClientRequest {
 
 @available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 extension HTTPClientRequest.Prepared {
-    init(_ request: HTTPClientRequest, dnsOverride: [String: String] = [:]) throws {
+    init(
+        _ request: HTTPClientRequest,
+        tracing: HTTPClient.TracingConfiguration? = nil,
+        dnsOverride: [String: String] = [:]
+    ) throws {
         guard !request.url.isEmpty, let url = URL(string: request.url) else {
             throw HTTPClientError.invalidURL
         }
@@ -54,6 +60,12 @@ extension HTTPClientRequest.Prepared {
 
         var headers = request.headers
         headers.addHostIfNeeded(for: deconstructedURL)
+        if let tracer = tracing?.tracer,
+            let context = ServiceContext.current
+        {
+            tracer.inject(context, into: &headers, using: HTTPHeadersInjector.shared)
+        }
+
         let metadata = try headers.validateAndSetTransportFraming(
             method: request.method,
             bodyLength: .init(request.body)

Tests/AsyncHTTPClientTests/HTTPClientTracingTests.swift

@@ -12,7 +12,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-@_spi(Tracing) import AsyncHTTPClient  // NOT @testable - tests that need @testable go into HTTPClientInternalTests.swift
+@_spi(Tracing) @testable import AsyncHTTPClient
 import Atomics
 import InMemoryTracing
 import Logging
@@ -147,4 +147,16 @@ final class HTTPClientTracingTests: XCTestCaseHTTPClientTestsBaseClass {
         XCTAssertTrue(span.errors.isEmpty, "Should have recorded error")
         XCTAssertEqual(span.attributes.get(client.tracing.attributeKeys.responseStatusCode), 404)
     }
+
+    func testTrace_preparedHeaders_include_fromSpan() async throws {
+        let url = self.defaultHTTPBinURLPrefix + "404-does-not-exist"
+        let request = HTTPClientRequest(url: url)
+
+        try tracer.withSpan("operation") { span in
+            let prepared = try HTTPClientRequest.Prepared(request, tracing: self.client.tracing)
+            XCTAssertTrue(prepared.head.headers.count > 2)
+            XCTAssertTrue(prepared.head.headers.contains(name: "in-memory-trace-id"))
+            XCTAssertTrue(prepared.head.headers.contains(name: "in-memory-span-id"))
+        }
+    }
 }