count redeemed/auto-deleted channels in prometheus

context

• when we were first building the auto-delete feature, it was helpful to be notified when people redeemed their channels and/or channels were deleted (1) to make sure the timing aspects of the feature worked as expected, (2) to get a sense of what percentage of people redeemed their channels
• now that we know the feature works, the notifications deliver less value. since we prompt admins to delete their channels more frequently (every week), we also get a lot of low-value notifications, leading to alert fatigue
• this MR would replace notifications with a prometheus counter metric, allowing us to track redemptions vs. deletions over time without wasting alerts to do so

behavior

GIVEN a channel scheduled for auto-deletion due to lack of use

• WHEN the admin redeems the channel
• THEN the system increments a channel_redeemed counter

GIVEN a channel scheduled for auto-deletion due to lack of use

• WHEN nobody redeems the channel and it is deleted
• THEN they system increments a channel_auto_deleted counter

WHEN a maintainer logs into our grafana dash

• THEN they will see a graph comparing redemptions to auto-deletions over time