[o11y] Avoid cloning tail event when dispatching it to single tail worker

src/workerd/io/trace-stream.c++

@@ -1053,16 +1053,23 @@ struct TailStreamWriterState {
       actives.truncate(activeEnd - actives.begin());
     }
 
+    // We do not expect any events after the outcome.
+    bool isClosing = event.event.is<Outcome>();
     // Deliver the event to the queue and make sure we are processing.
     for (auto& active: actives) {
-      active->queue.push(event.clone());
+      // Optimization: Elide copy for last tail worker, helpful for common case of only one STW
+      // being present.
+      if (&active == &actives.back()) {
+        active->queue.push(kj::mv(event));
+      } else {
+        active->queue.push(event.clone());
+      }
       if (!active->pumping) {
         waitUntilTasks.add(pump(kj::addRef(*active)));
       }
     }
 
-    // We do not expect any events after the outcome.
-    return event.event.is<Outcome>();
+    return isClosing;
   }
 
   // Delivers the queued tail events to a streaming tail worker.

src/workerd/io/trace.h

@@ -581,9 +581,6 @@ struct Attribute final {
   explicit Attribute(kj::ConstString name, Value&& value);
   explicit Attribute(kj::ConstString name, Values&& values);
 
-  template <AttributeValue V>
-  explicit Attribute(kj::ConstString name, V v): Attribute(kj::mv(name), Value(kj::mv(v))) {}
-
   template <AttributeValue V>
   explicit Attribute(kj::ConstString name, kj::Array<V> vals)
       : Attribute(kj::mv(name), KJ_MAP(v, vals) { return Value(kj::mv(v)); }) {}

src/workerd/io/tracer.c++

@@ -572,8 +572,7 @@ void WorkerTracer::setJsRpcInfo(const tracing::InvocationSpanContext& context,
 
   KJ_IF_SOME(writer, maybeTailStreamWriter) {
     auto tag = tracing::Attribute("jsrpc.method"_kjc, methodName.clone());
-    kj::Array<tracing::Attribute> attrs(&tag, 1, kj::NullArrayDisposer::instance);
-    writer->report(context, kj::mv(attrs), timestamp);
+    writer->report(context, kj::arr(kj::mv(tag)), timestamp);
   }
 }