Delete entry callback for reporter

[surik]

The reporter has exometer_newentry/2 callback but doesn't have exometer_deleteentry/2. It would be useful to have a callback about deleting an entry. What do you think?

[uwiger]

Seems reasonable, although I don't know off-hand if any of the usual reporting backends could make use of it. Can you offer a concrete use case?

[surik]

It can be used for unsubscribe metrics and release reporter resources.