repo-updater: Expose current rate limit settings per external service

Created by: ryanslade

It can be accessed via our existing instrumentation area:

Site Admin -> Instrumentation -> Repo Updater -> Rate Limiter State

It only shows the current Burst and Limit value as JSON

For example:

{
  "extsvc:github:1": {
    "Burst": 1,
    "Limit": 1.3888888888888888
  },
  "extsvc:github:2": {
    "Burst": 1,
    "Limit": 1.3888888888888888
  }
}

Test Plan

Tested locally