feat: Add diagnostics library for tracking stats/counters

Add a generic library that can be used to track counters, or elapsed time (e.g. how long it takes on average to connect to mqtt).  This is a copy of https://github.com/allenporter/python-google-nest-sdm/blob/main/google_nest_sdm/diagnostics.py

This is an initial pass to add a few initial example metrics for MQTT, but we can add more as we need fine grained details in diagnostics.

Author: allenporter

roborock/mqtt/roborock_session.py

@@ -240,6 +240,7 @@ async def _mqtt_client(self, params: MqttParams) -> aiomqtt.Client:
                 async with self._client_lock:
                     self._client = client
                     for topic in self._client_subscribed_topics:
+                        self._diagnostics.increment("resubscribe")
                         _LOGGER.debug("Re-establishing subscription to topic %s", topic)
                         # TODO: If this fails it will break the whole connection. Make
                         # this retry again in the background with backoff.
@@ -321,6 +322,7 @@ def delayed_unsub():
             unsub()  # Remove the callback from CallbackMap
             # If no more callbacks for this topic, start idle timer
             if not self._listeners.get_callbacks(topic):
+                self._diagnostics.increment("unsubscribe_idle_start")
                 schedule_unsubscribe()
             else:
                 _LOGGER.debug("Unsubscribing topic %s, still have active callbacks", topic)