message: [RunHandler] Add tracing event when a session waits for a free handler.
diff: @@ -887,9 +887,17 @@ class RunHandlerPool::Impl {
  RunHandler::Impl* handler_impl;
  {
  mutex_lock l(mu_);
  + if (free_handlers_.empty()) {
  + profiler::TraceMe activity(
  + [&] {
  + return strings::StrCat("WaitingForHandler#step_id=", step_id,
  + "#");
  + },
  + profiler::TraceMeLevel::kInfo);
  while (free_handlers_.empty()) {
  one_handler_free_.wait(l);
  }
  + }
  // Remove the last entry from free_handlers_ and add to the end of
  // sorted_active_handlers_.
  handler_impl = free_handlers_.back();
  