[REVERT THIS] Track some metrics to see how big this gets

synapse/storage/databases/main/events_worker.py

@@ -108,6 +108,10 @@
     "synapse_event_fetch_ongoing",
     "The number of event fetchers that are running",
 )
+event_metadata_cache_size_gauge = Gauge(
+    "synapse_event_metadata_cache_size",
+    "The size of event metadata cache in number of entries",
+)
 
 
 class InvalidEventError(Exception):
@@ -329,6 +333,11 @@ def get_chain_id_txn(txn: Cursor) -> int:
 
         self._un_partial_stated_events_stream_id_gen: AbstractStreamIdGenerator
 
+        # This is a crap way to track metrics, but will do for this experiment.
+        self.track_metadata_size_loop = self._clock.looping_call(
+            self.track_metadata_size, 1000
+        )
+
         if isinstance(database.engine, PostgresEngine):
             self._un_partial_stated_events_stream_id_gen = MultiWriterIdGenerator(
                 db_conn=db_conn,
@@ -351,6 +360,13 @@ def get_chain_id_txn(txn: Cursor) -> int:
                 "stream_id",
             )
 
+    def track_metadata_size(self) -> None:
+        # Get the total number of entries
+        size = len(self._event_metadata)
+        for room_id in self._event_metadata:
+            size += len(self._event_metadata[room_id].events)
+        event_metadata_cache_size_gauge.set(size)
+
     def get_un_partial_stated_events_token(self, instance_name: str) -> int:
         return (
             self._un_partial_stated_events_stream_id_gen.get_current_token_for_writer(