Add tracing spans for output gate wait

Author: shrima-cf

src/workerd/io/io-gate.c++

@@ -324,10 +324,11 @@ kj::Own<kj::PromiseFulfiller<void>> OutputGate::lock() {
   return kj::mv(paf.fulfiller);
 }
 
-kj::Promise<void> OutputGate::wait() {
+kj::Promise<void> OutputGate::wait(SpanParent parentSpan) {
   hooks.outputGateWaiterAdded();
+  SpanBuilder waitSpan = parentSpan.newChild("output_gate_lock_wait"_kjc);
   return pastLocksPromise.addBranch().attach(
-      kj::defer([this]() { hooks.outputGateWaiterRemoved(); }));
+      kj::defer([this]() { hooks.outputGateWaiterRemoved(); }), kj::mv(waitSpan));
 }
 
 kj::Promise<void> OutputGate::onBroken() {

src/workerd/io/io-gate.h

@@ -275,7 +275,7 @@ class OutputGate {
 
   // Wait until all preceding locks are released. The wait will not be affected by any future
   // call to `lockWhile()`.
-  kj::Promise<void> wait();
+  kj::Promise<void> wait(SpanParent parentSpan = nullptr);
 
   // Rejects if and when calls to `wait()` become broken due to a failed lockWhile(). The actor
   // should be shut down in this case. This promise never resolves, only rejects.