Record time to status and request duration: first byte for:

* all requests
* by requests forwarded to local Rails API server
* by requests only that involve contacting federated clusters (parameterize by remote?)
* federated by requests only

If separating out request types is too hard, just add the basic request timings. served directly from postgres by controller (do we have any that do that yet?)
* by HTTP method
* by endpoint (/arvados/v1/collections, /arvados/v1/containers etc)
* by result (200, 404, etc)

Use prometheus client for Go.