Reland "OTel: add the custom services REST endpoint family"

This reverts commit a946dd146565e5b59c39ed143cb4492dd62361a5.

Revert submission 151814-revert-151589-TOYEQNWXAI

Reason for revert: Fix failing tests

Reverted changes: /q/submissionid:151814-revert-151589-TOYEQNWXAI

Change-Id: Iec162d4724145dd6750e8d8a9b9291f3a1e8b8d1