Use monotonic time instead of the time of day

This uses a monotonic time instead of the timeofday provided by the
Kernel.

The timeofday is subject to change, if an administrator changes it
manually, or is updated by NTP.

lib/gitlab/metrics/requests_rack_middleware.rb

@@ -66,14 +66,14 @@ module Gitlab
       def call(env)
         method = env['REQUEST_METHOD'].downcase
         method = 'INVALID' unless HTTP_METHODS.key?(method)
-        started = Time.now.to_f
+        started = Gitlab::Metrics::System.monotonic_time
         health_endpoint = health_endpoint?(env['PATH_INFO'])
         status = 'undefined'
 
         begin
           status, headers, body = @app.call(env)
 
-          elapsed = Time.now.to_f - started
+          elapsed = Gitlab::Metrics::System.monotonic_time - started
 
           if !health_endpoint && Gitlab::Metrics.record_duration_for_status?(status)
             RequestsRackMiddleware.http_request_duration_seconds.observe({ method: method }, elapsed)