Add metrics for len of new extremities persisted.

Of new events being persisted add metrics for total size of forward
extremities and number of unchanged, "stale" extremities.

synapse/storage/events.py

@@ -24,7 +24,7 @@ from six import iteritems, text_type
 from six.moves import range
 
 from canonicaljson import json
-from prometheus_client import Counter
+from prometheus_client import Counter, Histogram
 
 from twisted.internet import defer
 
@@ -74,6 +74,19 @@ state_delta_reuse_delta_counter = Counter(
     "synapse_storage_events_state_delta_reuse_delta", ""
 )
 
+# The number of forward extremities for each new event.
+forward_extremities_counter = Histogram(
+    "synapse_storage_events_forward_extremities_persisted", "",
+    buckets=(1, 2, 3, 5, 7, 10, 15, 20, 50, 100, 200, 500, "+Inf")
+)
+
+# The number of stale forward extremities for each new event. Stale extremities
+# are those that were in the previous set of extremities as well as the new.
+stale_forward_extremities_counter = Histogram(
+    "synapse_storage_events_stale_forward_extremities_persisted", "",
+    buckets=(0, 1, 2, 3, 5, 7, 10, 15, 20, 50, 100, 200, 500, "+Inf")
+)
+
 
 def encode_json(json_object):
     """
@@ -541,6 +554,8 @@ class EventsStore(
             and not event.internal_metadata.is_soft_failed()
         ]
 
+        latest_event_ids = set(latest_event_ids)
+
         # start with the existing forward extremities
         result = set(latest_event_ids)
 
@@ -564,6 +579,13 @@ class EventsStore(
         )
         result.difference_update(existing_prevs)
 
+        # We only update metrics for events that change forward extremities
+        # (e.g. we ignore backfill/outliers/etc)
+        if result != latest_event_ids:
+            forward_extremities_counter.observe(len(result))
+            stale = set(latest_event_ids) & result
+            stale_forward_extremities_counter.observe(len(stale))
+
         defer.returnValue(result)
 
     @defer.inlineCallbacks