Room Statistics (#4338)

changelog.d/4338.feature

docs/sample_config.yaml

synapse/api/constants.py

synapse/config/homeserver.py

synapse/config/stats.py

synapse/handlers/stats.py

synapse/server.py

synapse/storage/__init__.py

synapse/storage/events_worker.py

synapse/storage/roommember.py

synapse/storage/schema/delta/54/stats.sql

synapse/storage/state_deltas.py

synapse/storage/stats.py

tests/handlers/test_stats.py

tests/rest/client/v1/utils.py