monitor count of relayable messages sent per channel

Value

• we'd like to have situational awareness of how much load the app is under at any given time so that we can see if high load correlates with increased delivery latency
• since the instrumentation for this is a prerequisite to instrumentation for actually measuring message lag, let's do it first!

Behavior

GIVEN a running app

• THEN any friendo should be able to log into grafana and see 2 graphs
  • one with one line per channel which goes up or down according to how many relayable messages were sent over that channel
  • another with a single line which goes up or down according to how many relayable messages were sent through the system overall

Implementation details

• the spot where we probably want to stick this instrumentation is in dispatcher.index.relay