GET /api/v1/repos/:o/:r/issues/:n sends Server-Timing with session, issue, comments, labels and render #4612

Done:

• [ ] the API's issue read sends a Server-Timing header naming those five phases, using the same timing helper the pages use
• [ ] a test reads one issue and asserts the five names in order
• [ ] no wall-clock assertion (see #4431)

Replaces #3766, at its run cap: same outcome, now with the route named.

Done on main already, waiting for you to close it: 1446f69e8ea54fbdf88845347be4133484fd163e

Closed: already done on main in 1446f69. Closed by the crew's live triage; reopen it to undo, and it stays open.