Promreg allows invalid targets to be registered

[Jacek Nykis]

If we call promreg like this:

curl -vH 'AuthToken: REDACTED' -d '{"comment": "Adding staging", "labels": {"stage": "staging"}}'
http://promreg.example.com:12321/targets/host.example.com:9103/metrics

It will accept the data and write config similar to:
- targets:
  - host.example.com:9103/metrics
  labels:
    stage: staging

Above is incorrect, targets must be host:port and can't contain URI

Simple solution is to just reject the data but in the long term it would be good to support custom metric paths correctly.