Track new users in user statistics.

This makes the rows 'completed' so that the stats regenerator need not
touch them.

synapse/storage/registration.py

@@ -868,6 +868,17 @@ class RegistrationStore(
                 (user_id_obj.localpart, create_profile_with_displayname),
             )
 
+        if self.hs.config.stats_enabled:
+            # we create a new completed user statistics row
+
+            # we don't strictly need current_token since this user really can't
+            # have any state deltas before now (as it is a new user), but still,
+            # we include it for completeness.
+            current_token = self._get_max_stream_id_in_current_state_deltas_txn(txn)
+            self._update_stats_delta_txn(
+                txn, now, "user", user_id, {}, complete_with_stream_id=current_token
+            )
+
         self._invalidate_cache_and_stream(txn, self.get_user_by_id, (user_id,))
         txn.call_after(self.is_guest.invalidate, (user_id,))
 
@@ -1139,6 +1150,7 @@ class RegistrationStore(
             deferred str|None: A str representing a link to redirect the user
             to if there is one.
         """
+
         # Insert everything into a transaction in order to run atomically
         def validate_threepid_session_txn(txn):
             row = self._simple_select_one_txn(