[enterprise] Metric to split cloud policy fetch response by type

This only concerns the Cloud Policy Client.

Instead of recording the fetch result for a single type for policy
fetch (TYPE_POLICY_FETCH), here we split fetch in 3 subtypes for more
granularity, these types being: user, machine, and machine base64.

This will help us (iOS) to track fetches for User Policy as we've
recently launched the feature. Right now, machine level (CBCM) and
User Policy fetches are in the same bucket which makes monitoring
hard.

Bug: 339400099
Change-Id: I961e6744e1082cc6350dc434d65cf0c62102cb95
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5527758
Reviewed-by: Owen Min <zmin@chromium.org>
Commit-Queue: Vincent Boisselle <vincb@google.com>
Cr-Commit-Position: refs/heads/main@{#1299305}