Clarify the docs for observability (#579)

Sources/Hummingbird/Middleware/LogRequestMiddleware.swift

@@ -15,7 +15,12 @@
 import HTTPTypes
 import Logging
 
-/// Middleware outputting to log for every call to server
+/// Middleware outputting to log for every call to server.
+///
+/// Uses [Swift-Log](https://github.com/apple/swift-log) for logging the output.
+/// Swift-Log has a flexible backend, and will output to console by default.
+/// You can replace the Logging backend with other implementations.
+/// A list of implementations is available in the swift-log repository's README.
 public struct LogRequestsMiddleware<Context: RequestContext>: RouterMiddleware {
     /// Header filter
     public struct HeaderFilter: Sendable, ExpressibleByArrayLiteral {

Sources/Hummingbird/Middleware/MetricsMiddleware.swift

@@ -19,6 +19,11 @@ import Metrics
 ///
 /// Records the number of requests, the request duration and how many errors were thrown. Each metric has additional
 /// dimensions URI and method.
+///
+/// Uses [Swift-Metrics](https://github.com/apple/swift-metrics) for recording the metrics.
+/// Swift-Metrics has a flexible backend, which will need to be initialized before any metrics are recorded.
+///
+/// A list of implementations is available in the swift-log repository's README.
 public struct MetricsMiddleware<Context: RequestContext>: RouterMiddleware {
     public init() {}
 

Sources/Hummingbird/Middleware/TracingMiddleware.swift

@@ -23,6 +23,11 @@ import Tracing
 ///
 /// You may opt in to recording a specific subset of HTTP request/response header values by passing
 /// a set of header names.
+///
+/// Uses [Swift-Distributed-Tracing](https://github.com/apple/swift-distributed-tracing) for recording the traces.
+/// Swift-Distributed-Tracing has a flexible backend, which will need to be initialized before any traces are recorded.
+///
+/// A list of implementations is available in the swift-distributed-tracing repository's README.
 public struct TracingMiddleware<Context: RequestContext>: RouterMiddleware {
     private let headerNamesToRecord: Set<RecordingHeader>
     private let attributes: SpanAttributes?