feat: Gradually add metrics capabilities to Instrumentation::Base

Author: zvkemp

instrumentation/base/Appraisals

@@ -0,0 +1,6 @@
+appraise "base" do
+  remove_gem "opentelemetry-metrics-api"
+end
+
+appraise "metrics-api" do
+end

instrumentation/base/Gemfile

@@ -6,4 +6,6 @@
 
 source 'https://rubygems.org'
 
+gem 'opentelemetry-metrics-api'
+
 gemspec

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

@@ -69,8 +69,9 @@ class << self
           integer: ->(v) { v.is_a?(Integer) },
           string: ->(v) { v.is_a?(String) }
         }.freeze
+        SINGLETON_MUTEX = Thread::Mutex.new
 
-        private_constant :NAME_REGEX, :VALIDATORS
+        private_constant :NAME_REGEX, :VALIDATORS, :SINGLETON_MUTEX
 
         private :new
 
@@ -163,20 +164,62 @@ def option(name, default:, validate:)
         end
 
         def instance
-          @instance ||= new(instrumentation_name, instrumentation_version, install_blk,
-                            present_blk, compatible_blk, options)
+          @instance || SINGLETON_MUTEX.synchronize do
+            @instance ||= new(instrumentation_name, instrumentation_version, install_blk,
+                              present_blk, compatible_blk, options, instrument_configs)
+          end
+        end
+
+        if defined?(OpenTelemetry::Metrics)
+          %i[
+            counter
+            observable_counter
+            histogram
+            gauge
+            observable_gauge
+            up_down_counter
+            observable_up_down_counter
+          ].each do |instrument_kind|
+            define_method(instrument_kind) do |name, **opts, &block|
+              opts[:callback] ||= block
+              register_instrument(instrument_kind, name, **opts)
+            end
+          end
+
+          def register_instrument(kind, name, **opts)
+            @instrument_configs ||= {}
+
+            key = [kind, name]
+            if @instrument_configs.key?(key)
+              warn("Duplicate instrument configured for #{self}: #{key.inspect}")
+            else
+              @instrument_configs[key] = opts
+            end
+          end
+        else
+          def counter(*, **); end
+          def observable_counter(*, **); end
+          def histogram(*, **); end
+          def gauge(*, **); end
+          def observable_gauge(*, **); end
+          def up_down_counter(*, **); end
+          def observable_up_down_counter(*, **); end
         end
 
         private
 
-        attr_reader :install_blk, :present_blk, :compatible_blk, :options
+        attr_reader :install_blk, :present_blk, :compatible_blk, :options, :instrument_configs
 
         def infer_name
           @inferred_name ||= if (md = name.match(NAME_REGEX)) # rubocop:disable Naming/MemoizedInstanceVariableName
                                md['namespace'] || md['classname']
                              end
         end
 
+        def metrics_defined?
+          defined?(OpenTelemetry::Metrics)
+        end
+
         def infer_version
           return unless (inferred_name = infer_name)
 
@@ -189,13 +232,13 @@ def infer_version
         end
       end
 
-      attr_reader :name, :version, :config, :installed, :tracer
+      attr_reader :name, :version, :config, :installed, :tracer, :meter, :instrument_configs
 
       alias installed? installed
 
       # rubocop:disable Metrics/ParameterLists
       def initialize(name, version, install_blk, present_blk,
-                     compatible_blk, options)
+                     compatible_blk, options, instrument_configs)
         @name = name
         @version = version
         @install_blk = install_blk
@@ -204,7 +247,9 @@ def initialize(name, version, install_blk, present_blk,
         @config = {}
         @installed = false
         @options = options
-        @tracer = OpenTelemetry::Trace::Tracer.new
+        @tracer = OpenTelemetry::Trace::Tracer.new # default no-op tracer
+        @meter = OpenTelemetry::Metrics::Meter.new if defined?(OpenTelemetry::Metrics::Meter) # default no-op meter
+        @instrument_configs = instrument_configs || {}
       end
       # rubocop:enable Metrics/ParameterLists
 
@@ -217,10 +262,21 @@ def install(config = {})
         return true if installed?
 
         @config = config_options(config)
+
+        @metrics_enabled = compute_metrics_enabled
+
+        if metrics_defined?
+          @metrics_instruments = {}
+          @instrument_mutex = Mutex.new
+        end
+
         return false unless installable?(config)
 
-        instance_exec(@config, &@install_blk)
         @tracer = OpenTelemetry.tracer_provider.tracer(name, version)
+        @meter = OpenTelemetry.meter_provider.meter(name, version: version) if metrics_enabled?
+
+        instance_exec(@config, &@install_blk)
+
         @installed = true
       end
 
@@ -261,8 +317,76 @@ def enabled?(config = nil)
         true
       end
 
+      # This is based on a variety of factors, and should be invalidated when @config changes.
+      # It should be explicitly set in `initialize` for now.
+      def metrics_enabled?
+        !!@metrics_enabled
+      end
+
+      # @api private
+      # ONLY yields if the meter is enabled.
+      def with_meter
+        yield @meter if metrics_enabled?
+      end
+
+      if defined?(OpenTelemetry::Metrics)
+        %i[
+          counter
+          observable_counter
+          histogram
+          gauge
+          observable_gauge
+          up_down_counter
+          observable_up_down_counter
+        ].each do |kind|
+          define_method(kind) do |name|
+            get_metrics_instrument(kind, name)
+          end
+        end
+      end
+
       private
 
+      def metrics_defined?
+        defined?(OpenTelemetry::Metrics)
+      end
+
+      def get_metrics_instrument(kind, name)
+        # FIXME: we should probably return *something*
+        # if metrics is not enabled, but if the api is undefined,
+        # it's unclear exactly what would be suitable.
+        # For now, there are no public methods that call this
+        # if metrics isn't defined.
+        return unless metrics_defined?
+
+        @metrics_instruments.fetch([kind, name]) do |key|
+          @instrument_mutex.synchronize do
+            @metrics_instruments[key] ||= create_configured_instrument(kind, name)
+          end
+        end
+      end
+
+      def create_configured_instrument(kind, name)
+        config = @instrument_configs[[kind, name]]
+
+        # FIXME: what is appropriate here?
+        if config.nil?
+          Kernel.warn("unconfigured instrument requested: #{kind} of '#{name}'")
+          return
+        end
+
+        # FIXME: some of these have different opts;
+        # should verify that they work before this point.
+        meter.public_send(:"create_#{kind}", name, **config)
+      end
+
+      def compute_metrics_enabled
+        return false unless defined?(OpenTelemetry::Metrics)
+        return false if metrics_disabled_by_env_var?
+
+        !!@config[:metrics] || metrics_enabled_by_env_var?
+      end
+
       # The config_options method is responsible for validating that the user supplied
       # config hash is valid.
       # Unknown configuration keys are not included in the final config hash.
@@ -317,13 +441,42 @@ def config_options(user_config)
       # will be OTEL_RUBY_INSTRUMENTATION_SINATRA_ENABLED. A value of 'false' will disable
       # the instrumentation, all other values will enable it.
       def enabled_by_env_var?
+        !disabled_by_env_var?
+      end
+
+      def disabled_by_env_var?
         var_name = name.dup.tap do |n|
           n.upcase!
           n.gsub!('::', '_')
           n.gsub!('OPENTELEMETRY_', 'OTEL_RUBY_')
           n << '_ENABLED'
         end
-        ENV[var_name] != 'false'
+        ENV[var_name] == 'false'
+      end
+
+      # Checks if this instrumentation's metrics are enabled by env var.
+      # This follows the conventions as outlined above, using `_METRICS_ENABLED` as a suffix.
+      # Unlike INSTRUMENTATION_*_ENABLED variables, these are explicitly opt-in (i.e.
+      # if the variable is unset, and `metrics: true` is not in the instrumentation's config,
+      # the metrics will not be enabled)
+      def metrics_enabled_by_env_var?
+        ENV.key?(metrics_env_var_name) && ENV[metrics_env_var_name] != 'false'
+      end
+
+      def metrics_disabled_by_env_var?
+        ENV[metrics_env_var_name] == 'false'
+      end
+
+      def metrics_env_var_name
+        @metrics_env_var_name ||=
+          begin
+            var_name = name.dup
+            var_name.upcase!
+            var_name.gsub!('::', '_')
+            var_name.gsub!('OPENTELEMETRY_', 'OTEL_RUBY_')
+            var_name << '_METRICS_ENABLED'
+            var_name
+          end
       end
 
       # Checks to see if the user has passed any environment variables that set options

instrumentation/base/opentelemetry-instrumentation-base.gemspec

@@ -29,6 +29,7 @@ Gem::Specification.new do |spec|
   spec.add_dependency 'opentelemetry-common', '~> 0.21'
   spec.add_dependency 'opentelemetry-registry', '~> 0.1'
 
+  spec.add_development_dependency 'appraisal', '~> 2.5'
   spec.add_development_dependency 'bundler', '~> 2.4'
   spec.add_development_dependency 'minitest', '~> 5.0'
   spec.add_development_dependency 'opentelemetry-test-helpers', '~> 0.3'