Add tracing spans for output gate hold

Author: shrima-cf

src/workerd/io/io-gate.h

@@ -271,7 +271,7 @@ class OutputGate {
   // If `promise` rejects, the exception will propagate to all future `wait()`s. If the returned
   // promise is canceled before completion, all future `wait()`s will also throw.
   template <typename T>
-  kj::Promise<T> lockWhile(kj::Promise<T> promise);
+  kj::Promise<T> lockWhile(kj::Promise<T> promise, SpanParent parentSpan = nullptr);
 
   // Wait until all preceding locks are released. The wait will not be affected by any future
   // call to `lockWhile()`.
@@ -303,8 +303,9 @@ class OutputGate {
 // inline implementation details
 
 template <typename T>
-kj::Promise<T> OutputGate::lockWhile(kj::Promise<T> promise) {
+kj::Promise<T> OutputGate::lockWhile(kj::Promise<T> promise, SpanParent parentSpan) {
   auto fulfiller = lock();
+  SpanBuilder lockSpan = parentSpan.newChild("output_gate_lock_hold"_kjc);
 
   if constexpr (std::is_void_v<T>) {
     promise = promise.exclusiveJoin(hooks.makeTimeoutPromise());