Log the room ID we're purging state for (#18625)

So we can see what we're deleting.

changelog.d/18625.misc

@@ -0,0 +1 @@
+Log the room ID we're purging state for.

synapse/storage/controllers/purge_events.py

@@ -34,6 +34,7 @@ from synapse.metrics.background_process_metrics import wrap_as_background_proces
 from synapse.storage.database import LoggingTransaction
 from synapse.storage.databases import Databases
 from synapse.types.storage import _BackgroundUpdates
+from synapse.util.stringutils import shortstr
 
 if TYPE_CHECKING:
     from synapse.server import HomeServer
@@ -167,6 +168,12 @@ class PurgeEventsStorageController:
                 break
 
             (room_id, groups_to_sequences) = next_to_delete
+
+            logger.info(
+                "[purge] deleting state groups for room %s: %s",
+                room_id,
+                shortstr(groups_to_sequences.keys(), maxitems=10),
+            )
             made_progress = await self._delete_state_groups(
                 room_id, groups_to_sequences
             )