Allow for reading and mutating a span name

Sources/Tracing/NoOpTracer.swift

@@ -29,7 +29,7 @@ public struct NoOpTracer: Tracer {
         file fileID: String,
         line: UInt
     ) -> Span {
-        NoOpSpan(baggage: baggage)
+        NoOpSpan(operationName: operationName, baggage: baggage)
     }
 
     public func forceFlush() {}
@@ -50,7 +50,18 @@ public struct NoOpTracer: Tracer {
         public let baggage: Baggage
         public let isRecording = false
 
-        public init(baggage: Baggage) {
+        public let _operationName: String
+        public var operationName: String {
+            get {
+                self._operationName
+            }
+            set {
+                // ignore
+            }
+        }
+
+        public init(operationName: String, baggage: Baggage) {
+            self._operationName = operationName
             self.baggage = baggage
         }
 

Sources/Tracing/Span.swift

@@ -30,6 +30,23 @@ public protocol Span: AnyObject, _SwiftTracingSendableSpan {
     /// The read-only `Baggage` of this `Span`, set when starting this `Span`.
     var baggage: Baggage { get }
 
+    /// Returns the name of the operation this span represents.
+    ///
+    /// The name may be changed during the lifetime of a `Span`, this change
+    /// may or may not impact the sampling decision and actually emitting the span,
+    /// depending on how a backend decides to treat renames.
+    ///
+    /// This can still be useful when, for example, we want to immediately start
+    /// span when receiving a request but make it more precise as handling of the request proceeds.
+    /// For example, we can start a span immediately when a request is received in a server,
+    /// and update it to reflect the matched route, if it did match one:
+    ///
+    /// - 1) Start span with basic path (e.g. `operationName = request.head.uri` during `withSpan`)
+    ///   - 2.1) "Route Not Found" -> Record error
+    ///   - 2.2) "Route Found" -> Rename to route (`/users/1` becomes `/users/:userID`)
+    /// - 3) End span
+    var operationName: String { get set }
+
     /// Set the status.
     /// - Parameter status: The status of this `Span`.
     func setStatus(_ status: SpanStatus)

Tests/TracingTests/DynamicTracepointTracerTests.swift

@@ -152,7 +152,7 @@ final class DynamicTracepointTestTracer: Tracer {
     {
         let tracepoint = TracepointID(function: function, fileID: fileID, line: line)
         guard self.shouldRecord(tracepoint: tracepoint) else {
-            return NoOpTracer.NoOpSpan(baggage: baggage)
+            return NoOpTracer.NoOpSpan(operationName: operationName, baggage: baggage)
         }
 
         let span = TracepointSpan(
@@ -230,14 +230,14 @@ final class DynamicTracepointTestTracer: Tracer {
 extension DynamicTracepointTestTracer {
     /// Only intended to be used in single-threaded testing.
     final class TracepointSpan: Tracing.Span {
-        private let operationName: String
         private let kind: SpanKind
 
         private var status: SpanStatus?
 
         private let startTime: DispatchWallTime
         private(set) var endTime: DispatchWallTime?
 
+        public var operationName: String
         private(set) var baggage: Baggage
         private(set) var isRecording: Bool = false
 

Tests/TracingTests/TestTracer.swift

@@ -96,14 +96,14 @@ extension Baggage {
 
 /// Only intended to be used in single-threaded testing.
 final class TestSpan: Span {
-    private let operationName: String
     private let kind: SpanKind
 
     private var status: SpanStatus?
 
     private let startTime: DispatchWallTime
     private(set) var endTime: DispatchWallTime?
 
+    var operationName: String
     let baggage: Baggage
 
     private(set) var events = [SpanEvent]() {

Tests/TracingTests/TracedLockTests.swift

@@ -98,14 +98,14 @@ private final class TracedLockPrintlnTracer: Tracer {
         Carrier == Extract.Carrier {}
 
     final class TracedLockPrintlnSpan: Span {
-        private let operationName: String
         private let kind: SpanKind
 
         private var status: SpanStatus?
 
         private let startTime: DispatchWallTime
         private(set) var endTime: DispatchWallTime?
 
+        var operationName: String
         let baggage: Baggage
 
         private var links = [SpanLink]()