Runtime Metrics API

It would be nice if Daphne could provide an api for runtime metrics for things like messages/active connections.

I imagine this could be built from 2 api's, one providing a basic http api we could hit and get the current active number of connections (in our case websocket) for that daphne instance.

Secondly a plugin/interface we could hook into to emit runtime events, we'd like to emit statsd events for connections opened/messages sent/closed etc, but I would imagine there are other metric systems people would use, so a generic stats interface / plugin system would be better than embedding statsd into daphne directly.

The issue names no files, tests, or entry points. Begin by mapping Daphne's HTTP and WebSocket server entry points and checking whether runtime events or extension interfaces already exist; then define the API and plugin boundaries, supported events, and tests needed to show that connection and message metrics are exposed without embedding StatsD.

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