Cherry pick PR #2667: Add Cobalt Telemetry for the Loader App

[cobalt-github-releaser-bot]

Refer to the original PR: https://github.com/youtube/cobalt/pull/2667

A LoaderAppMetrics Starboard extension is added so that measurements taken by the Loader App can be reliably accessed by Cobalt once loaded. This PR just adds one metric for now, for b/329458881, but this pattern and extension can be used for b/329445690 and other Loader App metrics.

b/329458881

Change-Id: I940d07b058d197afa2ad8b1160eba469f1055f8a

[cobalt-github-releaser-bot]

[!IMPORTANT] There were merge conflicts while cherry picking! Check out cherry-pick-24.lts.1+-2667 and fix the conflicts before proceeding. Check the log at https://github.com/youtube/cobalt/actions/runs/8525024517 for details.

[hlwarriner]

There's a small merge conflict because https://github.com/youtube/cobalt/pull/2387 wasn't cherry picked to 24.lts.1+. Let's see if we can just CP that PR, first.

[kaidokert]

Is this one still relevant ? Especially in case we have to do an unscheduled C24 release at any time ?

[hlwarriner]

Is this one still relevant ? Especially in case we have to do an unscheduled C24 release at any time ?

I think we decided not to cherry pick this after all since there are some changes needed by partners to wire up the Starboard extension in order to get this working. Let me know if you disagree or remember differently @yuying-y @gbournou and I can open up a new PR to cherry pick this to 24 LTS.