[#456] Resolve "channel churn visible in alerts and metrics"

Closes #456 (closed)

context

• we want to make it more visible how quickly we both add and lose channels (and members) so we can make more informed decisions about (1) when to close channel requests under load and (2) how long to wait before letting people off the waiting list for new channels

changes

alerting

alert maintainers when channel destroyed (with count of available phone numbers and active channels)

metrics

add SYSTEM_LOAD prometheus counter with labels for:

• channel created
• channel destroyed
• member created
• member destroyed