Move histograms related to plus addresses into their own file.

The goal of this CL is to eventually move all plus-address-related
metrics into the plus_addresses histograms file. For M127, we will
keep emitting the old metric, too, to make lookup in the UMA dashboard
easier.

Bug: 346076280
Change-Id: I35cffe7da0e6b78cba675792218aa31300d20433
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5611279


Reviewed-by: Norge Vizcay <vizcay@google.com>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Jan Keitel <jkeitel@google.com>
Cr-Commit-Position: refs/heads/main@{#1312634}