[Tracing] Default tracer to global bootstrapped tracer

The default tracer should be picked up at configuration creation time
from the bootstrapped one, so we don't have to manually configure it in
the simplest cases.

It is possible to disable it by explicitly setting `nil` on the tracing
configuration, even if the global tracer was bootstrapped.

Author: ktoso

Sources/AsyncHTTPClient/HTTPClient.swift

@@ -1095,7 +1095,11 @@ public final class HTTPClient: Sendable {
         package var attributeKeys: AttributeKeys
 
         public init() {
-            self._tracer = nil
+            if #available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *) {
+                self._tracer = InstrumentationSystem.tracer
+            } else {
+                self._tracer = nil
+            }
             self.attributeKeys = .init()
         }