Adds tracing documentation (#1039)

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Adds new Tracing chapter to document API introduced in
https://github.com/vapor/vapor/pull/3253

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docs/advanced/tracing.md

@@ -0,0 +1,83 @@
+# Tracing
+
+Tracing is a powerful tool for monitoring and debugging distributed systems. Vapor's tracing API allows developers to easily track request lifecycles, propagate metadata, and integrate with popular backends like OpenTelemetry.
+
+Vapor's tracing API is built on top of [swift-distributed-tracing](https://github.com/apple/swift-distributed-tracing), which means it is compatible with all of swift-distributed-tracing's [backend implementations](https://github.com/apple/swift-distributed-tracing/blob/main/README.md#tracing-backends).
+
+If you are unfamiliar with tracing and spans in Swift, review the [OpenTelemetry Trace documentation](https://opentelemetry.io/docs/concepts/signals/traces/) and [swift-distributed-tracing documentation](https://swiftpackageindex.com/apple/swift-distributed-tracing/main/documentation/tracing).
+
+## TracingMiddleware
+
+To automatically create a fully annotated span for each request, add the `TracingMiddleware` to your application.
+
+```swift
+app.middleware.use(TracingMiddleware())
+```
+
+To get accurate span measurements and ensure that tracing identifiers are passed along correctly to other services, add this middleware before other middlewares.
+
+## Adding Spans
+
+When adding spans to route handlers, it's ideal for them to be associated with the top-level request span. This is referred to as "span propagation" and can be handled in two different ways: automatic or manual.
+
+### Automatic Propagation
+
+Vapor has support to automatically propagate spans between middleware and route callbacks. To do so, set the `Application.traceAutoPropagation` property to true during configuration.
+
+```swift
+app.traceAutoPropagation = true
+```
+
+!!! note
+    Enabling auto-propagation may degrade performance on high-throughput APIs with minimal tracing needs, since request span metadata must be restored for every route handler regardless of whether spans are created.
+
+Then spans may be created in the route closure using the ordinary distributed tracing syntax.
+
+```swift
+app.get("fetchAndProcess") { req in
+    let result = try await fetch()
+    return try await withSpan("getNameParameter") { _ in
+        try await process(result)
+    }
+}
+```
+
+### Manual Propagation
+
+To avoid the performance implications of automatic propagation, you may manually restore span metadata where necessary. `TracingMiddleware` automatically sets a `Request.serviceContext` property which may be used directly in `withSpan`'s `context` parameter.
+
+```swift
+app.get("fetchAndProcess") { req in
+    let result = try await fetch()
+    return try await withSpan("getNameParameter", context: req.serviceContext) { _ in
+        try await process(result)
+    }
+}
+```
+
+To restore the span metadata without creating a span, use `ServiceContext.withValue`. This is valuable if you know that downstream async libraries emit their own tracing spans, and those should be nested underneath the parent request span.
+
+```swift
+app.get("fetchAndProcess") { req in
+    try await ServiceContext.withValue(req.serviceContext) {
+        try await fetch()
+        return try await process(result)
+    }
+}
+```
+
+## NIO Considerations
+
+Because `swift-distributed-tracing` uses [`TaskLocal properties`](https://developer.apple.com/documentation/swift/tasklocal) to propagate, you must manually re-restore the context whenever you cross `NIO EventLoopFuture` boundaries to ensure spans are linked correctly. **This is necessary regardless of whether automatic propagation is enabled**.
+
+```swift
+app.get("fetchAndProcessNIO") { req in
+    withSpan("fetch", context: req.serviceContext) { span in
+        fetchSomething().map { result in
+            withSpan("process", context: span.context) { _ in
+                process(result)
+            }
+        }
+    }
+}
+```

mkdocs.yml

@@ -578,6 +578,7 @@ nav:
   - Services: "advanced/services.md"
   - Request: "advanced/request.md"
   - APNS: "advanced/apns.md"
+  - Tracing: "advanced/tracing.md"
 - Security:
   - Authentication: "security/authentication.md"
   - Crypto: "security/crypto.md"