Make client compatible to API version 0.0.2

This change makes the ruby client JSON API 0.0.2 compatible. The
following changes were necessary

  * Rename type summary to histogram until API 0.0.3 is released
    and supported
  * Set correct Content-Type header

Minor changes to be compatible with the golang client label naming:

  * Rename http_request_latency_microseconds to
    http_request_durations_microseconds
  * Add method, path and code labels

Misc:

  * Add missing tests for middlewares