Add User Timings API marks and measures to the Repository frontend app

Context

While discussing LCP metrics in our Performance Round TableSource Code call, we noticed some unexpected differences explained by not having the same element trigger the LCP metric due to varying lengths of content.

We noticed we still haven't added any marks/measures from User Timings API like we've done in other apps.

These would allow us to discern better the changes in specific performance of our app.

Example

Where?	Screenshot
Browser console
Sitespeed report (metrics)

Proposal

• Add marks/measures to the file tree listing screen
• Add marks/measures to the blob view screen

Marks and measures

Type	Screen	Name of metric
Mark	File tree	Show first file
Mark	File tree	Show last file
Mark	File tree	Show commit
Measure	File tree	Rendering full list
Mark	Blob	Start blob content rendering
Mark	Blob	Finish blob content rendering
Mark	Blob	Show commit
Mark	Blob	Show codeowners
Measure	Blob	Rendering full blob content

(proposals, please feel free to tweak and edit and add/remove as necessary)