Update rack metric names

* Follow Prometheus naming conventions[0].
* Simplify metrics by adding response lables to the histogram.
* Use standard `http_request_duration_seconds_...` names for the histogram.

[0]: https://prometheus.io/docs/practices/naming/#metric-names

lib/gitlab/metrics/connection_rack_middleware.rb

@@ -6,19 +6,15 @@ module Gitlab
       end
 
       def self.rack_request_count
-        @rack_request_count ||= Gitlab::Metrics.counter(:rack_request, 'Rack request count')
-      end
-
-      def self.rack_response_count
-        @rack_response_count ||= Gitlab::Metrics.counter(:rack_response, 'Rack response count')
+        @rack_request_count ||= Gitlab::Metrics.counter(:http_requests_total, 'Rack request count')
       end
 
       def self.rack_uncaught_errors_count
-        @rack_uncaught_errors_count ||= Gitlab::Metrics.counter(:rack_uncaught_errors, 'Rack connections handling uncaught errors count')
+        @rack_uncaught_errors_count ||= Gitlab::Metrics.counter(:rack_uncaught_errors_total, 'Rack connections handling uncaught errors count')
       end
 
       def self.rack_execution_time
-        @rack_execution_time ||= Gitlab::Metrics.histogram(:rack_execution_time, 'Rack connection handling execution time',
+        @rack_execution_time ||= Gitlab::Metrics.histogram(:http_request_duration_seconds, 'Rack connection handling execution time',
                                                            {}, [0.05, 0.1, 0.25, 0.5, 0.7, 1, 1.5, 2, 2.5, 3, 5, 7, 10])
       end
 
@@ -30,14 +26,13 @@ module Gitlab
 
           status, headers, body = @app.call(env)
 
-          ConnectionRackMiddleware.rack_response_count.increment(method: method, status: status)
           [status, headers, body]
         rescue
           ConnectionRackMiddleware.rack_uncaught_errors_count.increment
           raise
         ensure
           elapsed = Time.now.to_f - started
-          ConnectionRackMiddleware.rack_execution_time.observe({}, elapsed)
+          ConnectionRackMiddleware.rack_execution_time.observe({method: method, status: status}, elapsed)
         end
       end
     end

spec/lib/gitlab/metrics/connection_rack_middleware_spec.rb

@@ -22,12 +22,6 @@ describe Gitlab::Metrics::ConnectionRackMiddleware do
         allow(app).to receive(:call).and_return([200, nil, nil])
       end
 
-      it 'increments response count with status label' do
-        expect(described_class).to receive_message_chain(:rack_response_count, :increment).with(include(status: 200, method: 'get'))
-
-        subject.call(env)
-      end
-
       it 'increments requests count' do
         expect(described_class).to receive_message_chain(:rack_request_count, :increment).with(method: 'get')
 
@@ -40,7 +34,7 @@ describe Gitlab::Metrics::ConnectionRackMiddleware do
           Timecop.freeze(execution_time.seconds)
         end
 
-        expect(described_class).to receive_message_chain(:rack_execution_time, :observe).with({}, execution_time)
+        expect(described_class).to receive_message_chain(:rack_execution_time, :observe).with({status: 200, method: 'get'}, execution_time)
 
         subject.call(env)
       end
@@ -49,11 +43,6 @@ describe Gitlab::Metrics::ConnectionRackMiddleware do
     context '@app.call throws exception' do
       let(:rack_response_count) { double('rack_response_count') }
 
-      before do
-        allow(app).to receive(:call).and_raise(StandardError)
-        allow(described_class).to receive(:rack_response_count).and_return(rack_response_count)
-      end
-
       it 'increments exceptions count' do
         expect(described_class).to receive_message_chain(:rack_uncaught_errors_count, :increment)