locate is subject to mlab-ns rate limiting

[stephen-soltesz]

The locate service should not be subject to mlab-ns rate limiting. The forwarded requests should preserve the user-agent and other metadata used to track rate limiting.

{
  "error": {
    "type": "nearest",
    "title": "Failed to lookup nearest machines",
    "status": 500
  }
}

[stephen-soltesz]

Originally reported in ooni/probe-engine#591

[stephen-soltesz]

Released https://github.com/m-lab/mlab-ns-rate-limit/releases/tag/v0.3.1

This should exempt locate from rate limiting until that logic is re-introduced in a locate-specific way.

[stephen-soltesz]

https://github.com/m-lab/dev-tracker/issues/587