feat: add experimental http.{server/client}.request.duration metrics to 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 +205,7 @@ def initialize(name, version, install_blk, present_blk,
         @installed = false
         @options = options
         @tracer = OpenTelemetry::Trace::Tracer.new
+        @meter = nil
       end
       # rubocop:enable Metrics/ParameterLists
 
@@ -222,6 +223,18 @@ def install(config = {})
         instance_exec(@config, &@install_blk)
         @tracer = OpenTelemetry.tracer_provider.tracer(name, version)
         @installed = true
+
+        if enable_metrics?(config) && defined?(OpenTelemetry.meter_provider) && OpenTelemetry.meter_provider.instance_of?(OpenTelemetry::SDK::Metrics::MeterProvider)
+          @meter = OpenTelemetry.meter_provider.meter(name, version: version)
+        end
+
+        initialize_metrics
+      end
+
+      # Each instrumentation can implement the initialize_metrics function to
+      # include the desired metrics instrument
+      def initialize_metrics
+        OpenTelemetry.logger.info 'Instrumentation should implement the function'
       end
 
       # Whether or not this instrumentation is installable in the current process. Will
@@ -261,6 +274,19 @@ def enabled?(config = nil)
         true
       end
 
+      # Whether this instrumentation has metrics is enabled. It first checks to see if it's enabled
+      # by an environment variable and will proceed to check if it's enabled
+      # by local config, if given. By default, it will be disabled.
+      #
+      # @param [optional Hash] config The local config
+      def enable_metrics?(config = nil)
+        env_value = metrics_env_var_value
+        return env_value == 'true' if %w[true false].include?(env_value)
+        return config[:metrics] if config&.key?(:metrics)
+
+        false
+      end
+
       private
 
       # The config_options method is responsible for validating that the user supplied
@@ -304,7 +330,7 @@ def config_options(user_config)
           h[option_name] = option[:default]
         end
 
-        dropped_config_keys = user_config.keys - validated_config.keys - [:enabled]
+        dropped_config_keys = user_config.keys - validated_config.keys - %i[enabled metrics]
         OpenTelemetry.logger.warn("Instrumentation #{name} ignored the following unknown configuration options #{dropped_config_keys}") unless dropped_config_keys.empty?
 
         validated_config
@@ -326,6 +352,21 @@ def enabled_by_env_var?
         ENV[var_name] != 'false'
       end
 
+      # Returns the value of the metrics environment variable. By convention, the
+      # environment variable will be the instrumentation name upper cased, with '::'
+      # replaced by underscores, OPENTELEMETRY shortened to OTEL_{LANG} and _METRICS_ENABLED appended.
+      # For example, the environment variable name for OpenTelemetry::Instrumentation::Sinatra
+      # will be OTEL_RUBY_INSTRUMENTATION_SINATRA_METRICS_ENABLED.
+      def metrics_env_var_value
+        var_name = name.dup.tap do |n|
+          n.upcase!
+          n.gsub!('::', '_')
+          n.gsub!('OPENTELEMETRY_', 'OTEL_RUBY_')
+          n << '_METRICS_ENABLED'
+        end
+        ENV.fetch(var_name, nil)
+      end
+
       # Checks to see if the user has passed any environment variables that set options
       # for instrumentation. By convention, the environment variable will be the name
       # of the instrumentation, uppercased, with '::' replaced by underscores,