Add some type hints to datastore

changelog.d/12248.misc

@@ -0,0 +1 @@
+Add missing type hints for storage.

mypy.ini

@@ -42,9 +42,6 @@ exclude = (?x)
    |synapse/storage/databases/main/cache.py
    |synapse/storage/databases/main/devices.py
    |synapse/storage/databases/main/event_federation.py
-   |synapse/storage/databases/main/group_server.py
-   |synapse/storage/databases/main/metrics.py
-   |synapse/storage/databases/main/monthly_active_users.py
    |synapse/storage/databases/main/push_rule.py
    |synapse/storage/databases/main/receipts.py
    |synapse/storage/databases/main/roommember.py
@@ -87,9 +84,6 @@ exclude = (?x)
    |tests/state/test_v2.py
    |tests/storage/test_background_update.py
    |tests/storage/test_base.py
-   |tests/storage/test_client_ips.py
-   |tests/storage/test_database.py
-   |tests/storage/test_event_federation.py
    |tests/storage/test_id_generators.py
    |tests/storage/test_roommember.py
    |tests/test_metrics.py