Consider sending non-expiry emails for permanent metrics

[chutten]

I think it would be helpful to send on, say, a yearly cadence a brace of emails to owners of permanent (non-expiring) metrics to remind them of which metrics they own.

I think it would also be helpful to provide a back-of-the-envelope calculation of the amount that those metrics cost Mozilla by being collected over the past twelve months, and will cost Mozilla by continuing to be collected over the next twelve months.

The action required of the owners to such an email would be to evaluate if there are any metrics on their list that have outlived their usefulness and can be removed or changed to have an expiry date in the next six months.

Thoughts?