histogram: Add junis@google.com to {ash,others}/OWNERS

This CL adds junis@google.com to {ash,others}/OWNERS. junis@ has
completed the Chromium Metrics Reviewing Quiz.

For individual metrics, this also adds junis@ as a histogram owner for
login related metrics replacing alemate@, who is no longer actively
working on ChromeOS.

Bug: None
Change-Id: I2d4fa51933cc0d7a38a970867c518f82213c3c5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5894058
Reviewed-by: Luc Nguyen <lucnguyen@google.com>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Commit-Queue: Jun Ishiguro <junis@google.com>
Cr-Commit-Position: refs/heads/main@{#1362283}