Add metrics to the receipts cache

synapse/metrics/metric.py

@@ -151,5 +151,12 @@ class CacheMetric(object):
     def inc_misses(self, *values):
         self.total.inc(*values)
 
+    def inc_hits_by(self, inc, *values):
+        self.hits.inc_by(inc, *values)
+        self.total.inc_by(inc, *values)
+
+    def inc_misses_by(self, inc, *values):
+        self.total.inc_by(inc, *values)
+
     def render(self):
         return self.hits.render() + self.total.render() + self.size.render()

synapse/storage/receipts.py

@@ -15,6 +15,7 @@
 
 from ._base import SQLBaseStore
 from synapse.util.caches.descriptors import cachedInlineCallbacks
+from synapse.util.caches import cache_counter, caches_by_name
 
 from twisted.internet import defer
 
@@ -305,6 +306,8 @@ class _RoomStreamChangeCache(object):
         self._room_to_key = {}
         self._cache = sorteddict()
         self._earliest_key = None
+        self.name = "ReceiptsRoomChangeCache"
+        caches_by_name[self.name] = self._cache
 
     @defer.inlineCallbacks
     def get_rooms_changed(self, store, room_ids, key):
@@ -321,6 +324,9 @@ class _RoomStreamChangeCache(object):
         else:
             result = room_ids
 
+        cache_counter.inc_hits_by(len(result), self.name)
+        cache_counter.inc_misses_by(len(room_ids) - len(result), self.name)
+
         defer.returnValue(result)
 
     @defer.inlineCallbacks