Instrument `get_metadata_for_events` for tracing (#13730)

When backfilling, `_get_state_ids_after_missing_prev_event` calls [`get_metadata_for_events`](https://github.com/matrix-org/synapse/blob/26bc26586b4b95d63ce7e453e9312469843f796e/synapse/handlers/federation_event.py#L1133). For `#matrix:matrix.org`, it's called with 77k `state_events` which means 77 calls to the database and takes 28 seconds.