Add a tab for Private Metrics in chrome://metrics-internals WebUI page

This commit adds a tab for Private Metrics WebUI in the Metrics
Internals WebUI page hosted at `chrome://private-metrics`. The new tab
is behind the feature flag `PrivateMetricsWebUI`.

Screenshots:
Feature Disabled: https://screenshot.googleplex.com/8dHhGKaGZo2wFYr.png
Feature Enabled: https://screenshot.googleplex.com/tFqc5i8G2VSZTLn.png

Bug: b:445106658
Change-Id: I07bc87b362c480590e4c0379e28231a30cf76c57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7072674
Reviewed-by: Colin Blundell <blundell@chromium.org>
Commit-Queue: Chirag Arora <heychirag@google.com>
Reviewed-by: Kamil Jarosz <kjarosz@google.com>
Cr-Commit-Position: refs/heads/main@{#1534214}
NOKEYCHECK=True
GitOrigin-RevId: 6b9fcbbe8bfece927db98e5d2da550664c1b5cc6