Enable struct Spans with reference semantics; optimal NoopSpan

**Motivation:**

We want a noop span to be NOT ref counted.

**Modifications:**

Allow spans to be value types through they must have reference semantics

**Result:**

Efficient noop spans

Author: ktoso

Sources/Tracing/NoOpTracer.swift

@@ -47,7 +47,7 @@ public struct NoOpTracer: LegacyTracerProtocol {
         // no-op
     }
 
-    public final class NoOpSpan: Span {
+    public struct NoOpSpan: Span {
         public let baggage: Baggage
         public var isRecording: Bool {
             false
@@ -57,7 +57,7 @@ public struct NoOpTracer: LegacyTracerProtocol {
             get {
                 "noop"
             }
-            set {
+            nonmutating set {
                 // ignore
             }
         }
@@ -78,11 +78,19 @@ public struct NoOpTracer: LegacyTracerProtocol {
             get {
                 [:]
             }
-            set {
+            nonmutating set {
                 // ignore
             }
         }
 
+        public func setAttribute(name: String, _ value: some SpanAttributeConvertible) {
+            // ignore
+        }
+
+        public func getAttribute(name: String) -> (SpanAttributeConvertible)? {
+            nil
+        }
+
         public func end(at time: DispatchWallTime) {
             // ignore
         }