[Serve] Provide backpressure on handle metrics push

[JoshKarpel]

Description

It would be nice to provide backpressure on handle metrics pushes to the Serve controller so that the controller does not become overloaded.

Relevant code is around these locations:

• ray/python/ray/serve/_private/metrics_utils.py

  Lines 48 to 73 in 9835610

  	async def metrics_task(self, name: str):
  	"""Periodically runs `task_func` every `interval_s` until `stop_event` is set.
  	If `task_func` raises an error, an exception will be logged.
  	"""
  	wait_for_stop_event = asyncio.create_task(self.stop_event.wait())
  	while True:
  	if wait_for_stop_event.done():
  	return
  	try:
  	self._tasks[name].task_func()
  	except Exception as e:
  	logger.exception(f"Failed to run metrics task '{name}': {e}")
  	sleep_task = asyncio.create_task(
  	self._async_sleep(self._tasks[name].interval_s)
  	)
  	await asyncio.wait(
  	[sleep_task, wait_for_stop_event],
  	return_when=asyncio.FIRST_COMPLETED,
  	)
  	if not sleep_task.done():
  	sleep_task.cancel()

• ray/python/ray/serve/_private/router.py

  Lines 258 to 265 in 9835610

  	self._controller_handle.record_handle_metrics.remote(
  	send_timestamp=time.time(),
  	deployment_id=self._deployment_id,
  	handle_id=self._handle_id,
  	actor_id=self._self_actor_id,
  	handle_source=self._handle_source,
  	**self._get_aggregated_requests(),
  	)

Currently the metrics push is fire-and-forget, and happens on a fixed interval whether or not the previous push has finished.

Use case

Our system is running a very large number of DeploymentHandles (see #44784 for more details). We've noticed that the Serve controller gets overloaded (>100% CPU usage) trying to accept all of the metrics pushes, which leads to an ever-increasing number of increasingly-stale record_handle_metrics tasks idle on the controller, which then eventually runs out of memory and crashes.

[JoshKarpel]

@zcin FYI with #45957, I suspect this won't be necessary from a performance/scalability perspective, though it may be good design to do it anyway