feat: 🦠 Metrics prototype for Rack instrumentation

instrumentation/base/lib/opentelemetry/instrumentation/base.rb

@@ -189,7 +189,7 @@ def infer_version
         end
       end
 
-      attr_reader :name, :version, :config, :installed, :tracer
+      attr_reader :name, :version, :config, :installed, :tracer, :meter
 
       alias installed? installed
 
@@ -205,6 +205,8 @@ def initialize(name, version, install_blk, present_blk,
         @installed = false
         @options = options
         @tracer = OpenTelemetry::Trace::Tracer.new
+        # check to see if the API is defined here because the config isn't available yet
+        @meter = OpenTelemetry::Metrics::Meter.new if defined?(OpenTelemetry::Metrics)
       end
       # rubocop:enable Metrics/ParameterLists
 
@@ -221,9 +223,20 @@ def install(config = {})
 
         instance_exec(@config, &@install_blk)
         @tracer = OpenTelemetry.tracer_provider.tracer(name, version)
+        install_meter
         @installed = true
       end
 
+      def install_meter
+        @meter = OpenTelemetry.meter_provider.meter(name, version: version) if metrics_enabled?
+      end
+
+      def metrics_enabled?
+        return @metrics_enabled if defined?(@metrics_enabled)
+
+        @metrics_enabled ||= defined?(OpenTelemetry::Metrics) && @config[:send_metrics]
+      end
+
       # Whether or not this instrumentation is installable in the current process. Will
       # be true when the instrumentation defines an install block, is not disabled
       # by environment or config, and the target library present and compatible.

instrumentation/rack/lib/opentelemetry/instrumentation/rack/instrumentation.rb

@@ -29,7 +29,9 @@ class Instrumentation < OpenTelemetry::Instrumentation::Base
         option :untraced_requests,        default: nil,   validate: :callable
         option :response_propagators,     default: [],    validate: :array
         # This option is only valid for applications using Rack 2.0 or greater
-        option :use_rack_events,          default: true, validate: :boolean
+        option :use_rack_events,          default: true,  validate: :boolean
+        # TODO: This option currently exclusively uses the event handler, should we support old and new Rack?
+        option :send_metrics,             default: false, validate: :boolean
 
         # Temporary Helper for Sinatra and ActionPack middleware to use during installation
         #

instrumentation/rack/lib/opentelemetry/instrumentation/rack/middlewares/event_handler.rb

@@ -203,6 +203,7 @@ def detach_context(request)
             token, span = request.env[OTEL_TOKEN_AND_SPAN]
             span.finish
             OpenTelemetry::Context.detach(token)
+            record_http_server_request_duration_metric(span)
           rescue StandardError => e
             OpenTelemetry.handle_error(exception: e)
           end
@@ -262,6 +263,43 @@ def create_span(parent_context, request)
             span.add_event('http.proxy.request.started', timestamp: request_start_time) unless request_start_time.nil?
             span
           end
+
+          # Metrics stuff
+          HTTP_SERVER_REQUEST_DURATION_ATTRS_FROM_SPAN = %w[http.method http.scheme http.route http.status_code http.host].freeze
+
+          def metrics_enabled?
+            OpenTelemetry::Instrumentation::Rack::Instrumentation.instance.metrics_enabled?
+          end
+
+          def meter
+            return unless metrics_enabled?
+
+            OpenTelemetry::Instrumentation::Rack::Instrumentation.instance.meter
+          end
+
+          def http_server_request_duration_histogram
+            return unless metrics_enabled?
+
+            @http_server_request_duration_histogram ||= meter.create_histogram(
+              'http.server.request.duration',
+              unit: 's',
+              description: 'Duration of HTTP server requests.'
+            )
+          end
+
+          def record_http_server_request_duration_metric(span)
+            return unless metrics_enabled?
+
+            # find span duration
+            # end - start / a billion to convert nanoseconds to seconds
+            duration = (span.end_timestamp - span.start_timestamp) / Float(10**9)
+            # glean attributes
+            attrs = span.attributes.select { |k, _v| HTTP_SERVER_REQUEST_DURATION_ATTRS_FROM_SPAN.include?(k) }
+            # set error
+            attrs['error.type'] = span.status.description if span.status.code == OpenTelemetry::Trace::Status::ERROR
+
+            http_server_request_duration_histogram.record(duration, attributes: attrs)
+          end
         end
       end
     end