Add network traffic annotation for Private Metrics

The annotation currently covers PUMA use cases, and will be updated in
the future when Private Metrics start supporting DWA, DKM, etc.

See go/puma-chrome-network-annotation.

Fix: b:463710662
Change-Id: Ia64cc7ee632efe3a5d0b3b283d9509ffc3e21b9c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7205526
Commit-Queue: Kamil Jarosz <kjarosz@google.com>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Matt Dembski <dembski@google.com>
Cr-Commit-Position: refs/heads/main@{#1550376}
NOKEYCHECK=True
GitOrigin-RevId: 3374f4d5d91c71ac9f662d5cbdc91e92f3b44d22