Allow for reading and mutating a span name, similar to otel spec

**Motivation:**

The feature frozen otel specification allows for renaming spans after
they have been started. This often is too late to impact sampling
decisions, however we still can use to to provide better names for spans
as time goes on.

**Modifications:**

add an operationName to the Span protocol.

A span has to be thread-safe already and we have mutating operations on
it already so it does not change the complexity requirements on
implementing a span.

**Result:**

Be closer to otel spec and more flexible in naming and renaming spans.

Resolves #48

Author: ktoso

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 {
+                _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]()