[EventEngine][Windows] Add shutdown grace period (#32713)

This adds a small wait for all timers to finish executing on
WindowsEventEngine shutdown. I have seen this wait code triggered on
rare occasions where either 10's of tests are running concurrently, or
in an environment where IPv6 is detected but disabled, and connection
cancellation takes seconds to complete. I have not gotten to the bottom
of the latter problem, but feel WindowsEventEngine destruction can
tolerate some small extra time.

src/core/lib/event_engine/windows/windows_engine.cc

@@ -109,15 +109,31 @@ WindowsEventEngine::WindowsEventEngine()
 WindowsEventEngine::~WindowsEventEngine() {
   GRPC_EVENT_ENGINE_TRACE("~WindowsEventEngine::%p", this);
   {
-    grpc_core::MutexLock lock(&task_mu_);
-    if (GRPC_TRACE_FLAG_ENABLED(grpc_event_engine_trace)) {
-      for (auto handle : known_handles_) {
-        gpr_log(GPR_ERROR,
-                "WindowsEventEngine:%p uncleared TaskHandle at shutdown:%s",
-                this, HandleToString<EventEngine::TaskHandle>(handle).c_str());
+    task_mu_.Lock();
+    if (!known_handles_.empty()) {
+      if (GRPC_TRACE_FLAG_ENABLED(grpc_event_engine_trace)) {
+        for (auto handle : known_handles_) {
+          gpr_log(GPR_ERROR,
+                  "WindowsEventEngine:%p uncleared TaskHandle at shutdown:%s",
+                  this,
+                  HandleToString<EventEngine::TaskHandle>(handle).c_str());
+        }
+      }
+      // Allow a small grace period for timers to be run before shutting down.
+      auto deadline =
+          timer_manager_.Now() + grpc_core::Duration::FromSecondsAsDouble(10);
+      while (!known_handles_.empty() && timer_manager_.Now() < deadline) {
+        if (GRPC_TRACE_FLAG_ENABLED(grpc_event_engine_trace)) {
+          GRPC_LOG_EVERY_N_SEC(1, GPR_DEBUG, "Waiting for timers. %d remaining",
+                               known_handles_.size());
+        }
+        task_mu_.Unlock();
+        absl::SleepFor(absl::Milliseconds(200));
+        task_mu_.Lock();
       }
     }
     GPR_ASSERT(GPR_LIKELY(known_handles_.empty()));
+    task_mu_.Unlock();
   }
   iocp_.Kick();
   iocp_worker_.WaitForShutdown();