MongoDB input plugin: Adding per Collection stats

[carlo4002]

Feature Request

Since #1466 we got the option to gather metrics for every database. Isn't it useful to have metrics per collection?

Proposal:

add the option gather_percollection_stats

Current behavior:

The metrics are collected at database level.

Desired behavior:

Have a better granularity to collect metrics by database and collection

Use case: [Why is this important (helps with prioritizing requests)]

Different collections are dedicated to different services therefore it is important to know which collection grows faster for example

[scottprichard]

We are using the mongo_db stats and would love this per collection metric!

[danielnelson]

Closed in #6137