channel churn visible in alerts and metrics

value

• under conditions of scarce capacity, we will shut down new channel creation and enqueue new channel requests in a waiting list (#447 (closed) , #455 )
• we would like to be able to monitor the rate at which channels are being auto-deleted, so we can make intelligent decisions about when it is safe to allow people in off the waitlist
• here we add monitoring and alerts to make churn visible to inform these decisions

behavior

alerts

GIVEN an instance with 100 active channels and 10 available (twilio) phone numbers: WHEN a channel is deleted

• THEN instance maintainers will receive a notification saying:

Channel deleted.
- 10 available phone numbers
- 99 active channels

monitoring

WHEN a maintainer visits the System Load (Channels) prometheus dashboard for a signalboost instance

• THEN they will see a Channels Deleted / day counter that shows how many channels were deleted in a given day (1-day step) over an arbitrary period
• THEN they will see a Channels Created / day counter that shows how many channels were created in a given day (1-day step) over an arbitrary period