Add number of documents as a size metric to a collection

[pboros]

Is your feature request related to a problem? Please describe.
Like the size metrics, we should have a metric counting the number of documents in a collection.

Describe the solution you'd like
At the time of updating or deleting a document, increment/decrement the counter.

Describe alternatives you've considered
A background task could query the count from FoundationDB but it's probably more expensive. Tally counter manipulation is cheap and asynchronous.