Instrument Unicorn with Ruby exporter

Gemfile

Gemfile.lock

config/gitlab.yml.example

config/initializers/1_settings.rb

config/initializers/8_metrics.rb

lib/gitlab/metrics/base_sampler.rb

lib/gitlab/metrics/connection_rack_middleware.rb

lib/gitlab/metrics/influx_sampler.rb

lib/gitlab/metrics/prometheus.rb

lib/gitlab/metrics/unicorn_sampler.rb

spec/initializers/8_metrics_spec.rb

spec/lib/gitlab/metrics/connection_rack_middleware_spec.rb

spec/lib/gitlab/metrics/influx_sampler_spec.rb

spec/lib/gitlab/metrics/unicorn_sampler_spec.rb