Introduce top level withSpan; reclaim Tracer for protocol

**Motivation:**
The TracerProtocol naming was annoyingly problematic and turns out we
can and should actually move the withSpan APIs as top level functions
which is typical Swift practice for such "global functionality".

This way we can `withSpan() {}` with less noise, and also gain back the
Tracer protocol name.

Author: ktoso

README.md

@@ -444,7 +444,7 @@ Spans form hierarchies with their parent spans, and end up being visualized usin
 The above trace is achieved by starting and ending spans in all the mentioned functions, for example, like this:
 
 ```swift
-let tracer: any TracerProtocol
+let tracer: any Tracer
 
 func makeDinner(context: LoggingContext) async throws -> Meal {
   tracer.withSpan(operationName: "makeDinner", context) {
@@ -538,8 +538,8 @@ func get(url: String, context: LoggingContext) {
 
 ## Instrument developers: Creating an instrument
 
-Creating an instrument means adopting the `InstrumentProtocol` protocol (or `TracerProtocol` in case you develop a tracer).
-`InstrumentProtocol` is part of the `Instrumentation` library & `Tracing` contains the `TracerProtocol` protocol.
+Creating an instrument means adopting the `InstrumentProtocol` protocol (or `Tracer` in case you develop a tracer).
+`InstrumentProtocol` is part of the `Instrumentation` library & `Tracing` contains the `Tracer` protocol.
 
 `InstrumentProtocol` has two requirements:
 
@@ -556,7 +556,7 @@ how to retrieve values from the `LoggingContext` and how to set values on it.
 
 When creating a tracer you need to create two types:
 
-1. Your tracer conforming to `TracerProtocol`
+1. Your tracer conforming to `Tracer`
 2. A span class conforming to `Span`
 
 > The `Span` conforms to the standard rules defined in [OpenTelemetry](https://github.com/open-telemetry/opentelemetry-specification/blob/v0.7.0/specification/trace/api.md#span), so if unsure about usage patterns, you can refer to this specification and examples referring to it.

Sources/Tracing/Docs.docc/InDepthGuide.md

@@ -188,7 +188,7 @@ Spans form hierarchies with their parent spans, and end up being visualized usin
 The above trace is achieved by starting and ending spans in all the mentioned functions, for example, like this:
 
 ```swift
-let tracer: any TracerProtocol
+let tracer: any Tracer
 
 func makeDinner(context: LoggingContext) async throws -> Meal {
   tracer.withSpan(operationName: "makeDinner", context) {

Sources/Tracing/InstrumentationSystem+Tracing.swift

@@ -22,9 +22,9 @@ extension InstrumentationSystem {
     /// tracing instrument as passed to the multiplex instrument. If none is found, a ``NoOpTracer`` is returned.
     ///
     /// - Returns: A ``Tracer`` if the system was bootstrapped with one, and ``NoOpTracer`` otherwise.
-    public static var tracer: any TracerProtocol {
-        let found: (any TracerProtocol)? =
-            (self._findInstrument(where: { $0 is (any TracerProtocol) }) as? (any TracerProtocol))
+    public static var tracer: any Tracer {
+        let found: (any Tracer)? =
+            (self._findInstrument(where: { $0 is (any Tracer) }) as? (any Tracer))
         return found ?? NoOpTracer()
     }
     #endif
@@ -35,9 +35,9 @@ extension InstrumentationSystem {
     /// tracing instrument as passed to the multiplex instrument. If none is found, a ``NoOpTracer`` is returned.
     ///
     /// - Returns: A ``Tracer`` if the system was bootstrapped with one, and ``NoOpTracer`` otherwise.
-    public static var legacyTracer: any LegacyTracerProtocol {
-        let found: (any LegacyTracerProtocol)? =
-            (self._findInstrument(where: { $0 is (any LegacyTracerProtocol) }) as? (any LegacyTracerProtocol))
+    public static var legacyTracer: any LegacyTracer {
+        let found: (any LegacyTracer)? =
+            (self._findInstrument(where: { $0 is (any LegacyTracer) }) as? (any LegacyTracer))
         return found ?? NoOpTracer()
     }
 }

Sources/Tracing/NoOpTracer.swift

@@ -18,7 +18,7 @@ import Dispatch
 
 /// Tracer that ignores all operations, used when no tracing is required.
 @available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *) // for TaskLocal Baggage
-public struct NoOpTracer: LegacyTracerProtocol {
+public struct NoOpTracer: LegacyTracer {
     public typealias TracerSpan = NoOpSpan
 
     public init() {}
@@ -91,7 +91,7 @@ public struct NoOpTracer: LegacyTracerProtocol {
 }
 
 #if swift(>=5.7.0)
-extension NoOpTracer: TracerProtocol {
+extension NoOpTracer: Tracer {
     public func startSpan<Clock: TracerClock>(
         _ operationName: String,
         baggage: @autoclosure () -> Baggage,