metrics: add a way to measure batch change size

Created by: malomarrec

Motivation

We are revamping pricing and gating, and we have an opportunity to reevaluate the gate on "you can create a batch change with 5 changesets for free". To do so, we'll need to set the right gate, which requires some data.

Proposal

We are already able to track average batch change size, but that doesn't inform the discussion enough. We need insights into the distribution of changesets. For privacy reason, we will not want to report the number of PRs in every change. Instead, we could report the number of batch changes with more than:

• 0 changesets
• 10 changesets
• 50 changesets
• 100 changesets
• 1000 changesets

We will also want to be able to tell if the behaviour is different on server side batch changes when it is available. That will be the case for all metrics, so this is not documented here but will be in a separate ticket.