Log internal HTTP requests

This restores the previous behavior of logging all internal HTTP requests.

Sample successful message:

{
  "duration_ms": 59.478,
  "level": "info",
  "method": "POST",
  "msg": "Finished HTTP request",
  "time": "2020-03-11T05:02:17Z",
  "url": "http://127.0.0.1:8080/gitlab/api/v4/internal/allowed"
}

Sample unsuccessful message:

{
  "duration_ms": 0.345,
  "error": "Post http://127.0.0.1:8080/gitlab/api/v4/internal/allowed: dial tcp 127.0.0.1:8080: connect: connection refused",
  "level": "error",
  "method": "POST",
  "msg": "Internal API unreachable",
  "time": "2020-03-11T05:02:55Z",
  "url": "http://127.0.0.1:8080/gitlab/api/v4/internal/allowed"
}

Part of gitlab#207916 (closed)