Add recordingSpan and currentSpan methods to TracerProtocol

**Motivation:**

Currently, it is only possible to implicitly work with the current span by transparently
creating a child span (using ServiceContext.current) under the hood. This is sufficient
for almost all use-cases, but does not work in case a piece of code wants to add an event
to the current span without having a handle on said span, e.g. if the span was created by a
library.

**Modifications:**

I added a `recordingSpan(identifiedBy context: ServiceContext) -> Span?` requirement to `TracerProtocol`. This way, `Tracer` implementations may look up and return a span identified by the data they stored in the provided `ServiceContext`. It's worth noting that this method is only intended for obtaining spans which are still recording as opposed to already ended ones that may not even be in memory anymore.
I also added a default implementation of this method to avoid introducing a breaking change.
On top of this new protocol requirement, I added an extension to `TracerProtocol` with sugar to obtain the current span based on the task-local `ServiceContext`.

**Result:**

Library authors and application developers are now able to look up the current recording span to interact with it by e.g. adding events and attributes.

Author: slashmo

Sources/Tracing/TracerProtocol.swift

@@ -59,6 +59,13 @@ public protocol Tracer: LegacyTracer {
         file fileID: String,
         line: UInt
     ) -> Self.Span
+
+    /// Retrieve the recording span for the given `ServiceContext`.
+    ///
+    /// - Note: This API does not enable look up of already finished spans.
+    /// - Parameter context: The context containing information that uniquely identifies the span being obtained.
+    /// - Returns: The span identified by the given `ServiceContext` in case it's still recording.
+    func recordingSpan(identifiedBy context: ServiceContext) -> Span?
 }
 
 @available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)  // for TaskLocal ServiceContext
@@ -106,6 +113,23 @@ extension Tracer {
             line: line
         )
     }
+
+    /// Attempt to retrieve the span for the given `ServiceContext`.
+    ///
+    /// - Parameter context: The context containing information that uniquely identifies the span being obtained.
+    /// - Returns: The span which created the given `ServiceContext`.
+    public func recordingSpan(identifiedBy context: ServiceContext) -> Span? {
+        nil
+    }
+
+    /// Attempt to retrieve the current recording span based on the task-local `ServiceContext`.
+    ///
+    /// - Returns: A span if one can be obtained via the task-local `ServiceContext`.
+    @available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
+    public func currentSpan() -> Span? {
+        guard let context = ServiceContext.current else { return nil }
+        return recordingSpan(identifiedBy: context)
+    }
 }
 
 // ==== ----------------------------------------------------------------------------------------------------------------