Rails app metrics provided by yabeda-rails gem

• rails_requests_total Total number of HTTP requests rails processed
• rails_request_duration_seconds Histogram of the response latency
• rails_view_runtime_seconds Histogram of the view rendering time
• rails_db_runtime_seconds Histogram of the activerecord execution time

zync example: https://github.com/3scale/zync/commit/e2cafb1575ba5e84f57740d8bfa56f0a43269fa0

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