https://github.com/discourse/discourse/blob/ecb098bcbb0aad9e7d3777e036e4eafb0cfece9b/lib/middleware/request_tracker.rb#L155-L161
https://github.com/discourse/discourse/blob/ecb098bcbb0aad9e7d3777e036e4eafb0cfece9b/config/nginx.sample.conf#L100-L100
Amount of time it takes a request to find its way from NGINX to the Unicorn worker process.