Run Prometheus on a different port, optionally. (#3274)

docs/metrics-howto.rst

synapse/app/_base.py

synapse/app/appservice.py

synapse/app/client_reader.py

synapse/app/event_creator.py

synapse/app/federation_reader.py

synapse/app/federation_sender.py

synapse/app/frontend_proxy.py

synapse/app/homeserver.py

synapse/app/media_repository.py

synapse/app/pusher.py

synapse/app/synchrotron.py

synapse/app/user_dir.py

synapse/config/server.py

synapse/metrics/__init__.py

synapse/metrics/resource.py