feat: add synchronous gauge

metrics_api/lib/opentelemetry/internal/proxy_instrument.rb

@@ -19,7 +19,7 @@ def initialize(kind, name, unit, desc, callable)
 
       def upgrade_with(meter)
         @delegate = case @kind
-                    when :counter, :histogram, :up_down_counter
+                    when :counter, :histogram, :up_down_counter, :gauge
                       meter.send("create_#{@kind}", @name, unit: @unit, description: @desc)
                     when :observable_counter, :observable_gauge, :observable_up_down_counter
                       meter.send("create_#{@kind}", @name, unit: @unit, description: @desc, callback: @callback)

metrics_api/lib/opentelemetry/internal/proxy_meter.rb

@@ -45,6 +45,7 @@ def create_instrument(kind, name, unit, description, callback)
           case kind
           when :counter then @delegate.create_counter(name, unit: unit, description: description)
           when :histogram then @delegate.create_histogram(name, unit: unit, description: description)
+          when :gauge then @delegate.create_gauge(name, unit: unit, description: description)
           when :up_down_counter then @delegate.create_up_down_counter(name, unit: unit, description: description)
           when :observable_counter then @delegate.create_observable_counter(name, unit: unit, description: description, callback: callback)
           when :observable_gauge then @delegate.create_observable_gauge(name, unit: unit, description: description, callback: callback)

metrics_api/lib/opentelemetry/metrics/instrument.rb

@@ -6,6 +6,7 @@
 
 require 'opentelemetry/metrics/instrument/counter'
 require 'opentelemetry/metrics/instrument/histogram'
+require 'opentelemetry/metrics/instrument/gauge'
 require 'opentelemetry/metrics/instrument/observable_counter'
 require 'opentelemetry/metrics/instrument/observable_gauge'
 require 'opentelemetry/metrics/instrument/observable_up_down_counter'

metrics_api/lib/opentelemetry/metrics/instrument/gauge.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+# Copyright The OpenTelemetry Authors
+#
+# SPDX-License-Identifier: Apache-2.0
+
+module OpenTelemetry
+  module Metrics
+    module Instrument
+      # No-op implementation of Gauge.
+      class Gauge
+        # Record the current value for the Gauge
+        #
+        # @param [Numeric] amount The current absolute value.
+        # @param [Hash{String => String, Numeric, Boolean, Array<String, Numeric, Boolean>}] attributes
+        #   Values must be non-nil and (array of) string, boolean or numeric type.
+        #   Array values must not contain nil elements and all elements must be of
+        #   the same basic type (string, numeric, boolean).
+        def record(amount, attributes: {}); end
+      end
+    end
+  end
+end

metrics_api/lib/opentelemetry/metrics/meter.rb

@@ -11,13 +11,14 @@ class Meter
       COUNTER = Instrument::Counter.new
       OBSERVABLE_COUNTER = Instrument::ObservableCounter.new
       HISTOGRAM = Instrument::Histogram.new
+      GAUGE = Instrument::Gauge.new
       OBSERVABLE_GAUGE = Instrument::ObservableGauge.new
       UP_DOWN_COUNTER = Instrument::UpDownCounter.new
       OBSERVABLE_UP_DOWN_COUNTER = Instrument::ObservableUpDownCounter.new
 
       NAME_REGEX = /\A[a-zA-Z][-.\w]{0,62}\z/
 
-      private_constant(:COUNTER, :OBSERVABLE_COUNTER, :HISTOGRAM, :OBSERVABLE_GAUGE, :UP_DOWN_COUNTER, :OBSERVABLE_UP_DOWN_COUNTER)
+      private_constant(:COUNTER, :OBSERVABLE_COUNTER, :HISTOGRAM, :GAUGE, :OBSERVABLE_GAUGE, :UP_DOWN_COUNTER, :OBSERVABLE_UP_DOWN_COUNTER)
 
       DuplicateInstrumentError = Class.new(OpenTelemetry::Error)
       InstrumentNameError = Class.new(OpenTelemetry::Error)
@@ -37,6 +38,24 @@ def create_histogram(name, unit: nil, description: nil)
         create_instrument(:histogram, name, unit, description, nil) { HISTOGRAM }
       end
 
+      # Gauge is an synchronous Instrument which reports non-additive value(s)
+      #
+      # With this api call:
+      #
+      #   meter.create_gauge("cpu.frequency",
+      #                       description: "the real-time CPU clock speed",
+      #                       unit: "ms")
+      #
+      #
+      # @param name [String] the name of the gauge.
+      # @param unit [optional String] an optional string provided by user.
+      # @param description [optional String] an optional free-form text provided by user.
+      #
+      # @return [nil] after creation of gauge, it will be stored in instrument_registry
+      def create_gauge(name, unit: nil, description: nil)
+        create_instrument(:gauge, name, unit, description, nil) { GAUGE }
+      end
+
       def create_up_down_counter(name, unit: nil, description: nil)
         create_instrument(:up_down_counter, name, unit, description, nil) { UP_DOWN_COUNTER }
       end

metrics_sdk/lib/opentelemetry/sdk/metrics/instrument.rb

@@ -20,3 +20,4 @@ module Instrument
 require 'opentelemetry/sdk/metrics/instrument/observable_gauge'
 require 'opentelemetry/sdk/metrics/instrument/observable_up_down_counter'
 require 'opentelemetry/sdk/metrics/instrument/up_down_counter'
+require 'opentelemetry/sdk/metrics/instrument/gauge'

metrics_sdk/lib/opentelemetry/sdk/metrics/instrument/gauge.rb

@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+
+# Copyright The OpenTelemetry Authors
+#
+# SPDX-License-Identifier: Apache-2.0
+
+module OpenTelemetry
+  module SDK
+    module Metrics
+      module Instrument
+        # {Gauge} is the SDK implementation of {OpenTelemetry::Metrics::Gauge}.
+        class Gauge < OpenTelemetry::SDK::Metrics::Instrument::SynchronousInstrument
+          # Returns the instrument kind as a Symbol
+          #
+          # @return [Symbol]
+          def instrument_kind
+            :gauge
+          end
+
+          # Record the absolute value of the Gauge.
+          #
+          # @param [numeric] value The current absolute value.
+          # @param [Hash{String => String, Numeric, Boolean, Array<String, Numeric, Boolean>}] attributes
+          #   Values must be non-nil and (array of) string, boolean or numeric type.
+          #   Array values must not contain nil elements and all elements must be of
+          #   the same basic type (string, numeric, boolean).
+          def record(value, attributes: {})
+            # TODO: When the metrics SDK stabilizes and is merged into the main SDK,
+            # we can leverage the SDK Internal validation classes to enforce this:
+            # https://github.com/open-telemetry/opentelemetry-ruby/blob/6bec625ef49004f364457c26263df421526b60d6/sdk/lib/opentelemetry/sdk/internal.rb#L47
+            update(value, attributes)
+            nil
+          rescue StandardError => e
+            OpenTelemetry.handle_error(exception: e)
+            nil
+          end
+
+          private
+
+          # TODO: replace the default aggregation to LastValue
+          def default_aggregation
+            OpenTelemetry::SDK::Metrics::Aggregation::Sum.new
+          end
+        end
+      end
+    end
+  end
+end

metrics_sdk/lib/opentelemetry/sdk/metrics/meter.rb

@@ -38,6 +38,7 @@ def create_instrument(kind, name, unit, description, callback)
             case kind
             when :counter then OpenTelemetry::SDK::Metrics::Instrument::Counter.new(name, unit, description, @instrumentation_scope, @meter_provider)
             when :observable_counter then OpenTelemetry::SDK::Metrics::Instrument::ObservableCounter.new(name, unit, description, callback, @instrumentation_scope, @meter_provider)
+            when :gauge then OpenTelemetry::SDK::Metrics::Instrument::Gauge.new(name, unit, description, @instrumentation_scope, @meter_provider)
             when :histogram then OpenTelemetry::SDK::Metrics::Instrument::Histogram.new(name, unit, description, @instrumentation_scope, @meter_provider)
             when :observable_gauge then OpenTelemetry::SDK::Metrics::Instrument::ObservableGauge.new(name, unit, description, callback, @instrumentation_scope, @meter_provider)
             when :up_down_counter then OpenTelemetry::SDK::Metrics::Instrument::UpDownCounter.new(name, unit, description, @instrumentation_scope, @meter_provider)

metrics_sdk/test/opentelemetry/sdk/metrics/instrument/gauge_test.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+# Copyright The OpenTelemetry Authors
+#
+# SPDX-License-Identifier: Apache-2.0
+
+require 'test_helper'
+
+describe OpenTelemetry::SDK::Metrics::Instrument::Gauge do
+  let(:metric_exporter) { OpenTelemetry::SDK::Metrics::Export::InMemoryMetricPullExporter.new }
+  let(:meter) { OpenTelemetry.meter_provider.meter('test') }
+  let(:gauge) { meter.create_gauge('gauge', unit: 'smidgen', description: 'a small amount of something') }
+
+  before do
+    reset_metrics_sdk
+    OpenTelemetry::SDK.configure
+    OpenTelemetry.meter_provider.add_metric_reader(metric_exporter)
+  end
+
+  it 'gauge should count -2' do
+    gauge.record(-2, attributes: { 'foo' => 'bar' })
+    metric_exporter.pull
+    last_snapshot = metric_exporter.metric_snapshots.last
+
+    _(last_snapshot[0].name).must_equal('gauge')
+    _(last_snapshot[0].unit).must_equal('smidgen')
+    _(last_snapshot[0].description).must_equal('a small amount of something')
+    _(last_snapshot[0].instrumentation_scope.name).must_equal('test')
+    _(last_snapshot[0].data_points[0].attributes).must_equal('foo' => 'bar')
+    _(last_snapshot[0].data_points[0].value).must_equal(-2)
+    _(last_snapshot[0].aggregation_temporality).must_equal(:delta)
+  end
+end