[mongo] Adds metrics for collections

[gmmeyer]

What does this PR do?

This PR adds some additional metrics from collections. It will now collect the following metrics from each collection (from the database that you're collecting from):

mongodb.collection.indexSizes
mongodb.collection.size
mongodb.collection.avgObjSize
mongodb.collection.count
mongodb.collection.capped
mongodb.collection.max
mongodb.collection.maxSize
mongodb.collection.storageSize
mongodb.collection.nindexes

Motivation

Collection level metrics, like the amount of space that a collection is taking up, or the average object size, scoped to a specific collection, can be very important. If a certain collection is too large, maybe it should be split up. If the indexes on a collection are getting much, much larger than the others, maybe you want to split the collection. There are a lot of these things that can be important.

Testing Guidelines

It tests to see whether it works in the most basic sense.

[remh]

Does it need to be in the default metrics as well ? Looks like it's processed differently, no ?

[remh]

We should not do it for every collection by default. Can you do something similar to the postgres check where we list the tables we want more metrics on (here it would be the collections not the tables)

[degemer]

👍