docs: add documentation for Metrics API instruments

metrics_api/lib/opentelemetry/metrics/meter.rb

@@ -27,26 +27,123 @@ def initialize
         @instrument_registry = {}
       end
 
+      # {https://opentelemetry.io/docs/specs/otel/metrics/api/#counter Counter} is a synchronous Instrument which supports non-negative increments.
+      #
+      # With this api call:
+      #
+      #   exception_counter = meter.create_counter("exceptions",
+      #                                            description: "number of exceptions caught",
+      #                                            unit: 's')
+      #
+      # @param name [String] the name of the counter
+      # @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 counter, it will be stored in instrument_registry
       def create_counter(name, unit: nil, description: nil)
         create_instrument(:counter, name, unit, description, nil) { COUNTER }
       end
 
+      # Histogram is a synchronous Instrument which can be used to report arbitrary values that are likely
+      # to be statistically meaningful. It is intended for statistics such as histograms,
+      # summaries, and percentiles.
+      #
+      # With this api call:
+      #
+      #   http_server_duration = meter.create_histogram("http.server.duration",
+      #                                                 description: "measures the duration of the inbound HTTP request",
+      #                                                 unit: "s")
+      #
+      # @param name [String] the name of the histogram
+      # @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 histogram, it will be stored in instrument_registry
       def create_histogram(name, unit: nil, description: nil)
         create_instrument(:histogram, name, unit, description, nil) { HISTOGRAM }
       end
 
+      # UpDownCounter is a synchronous Instrument which supports increments and decrements.
+      #
+      # With this api call:
+      #
+      #   items_counter = meter.create_up_down_counter("store.inventory",
+      #                                                description: "the number of the items available",
+      #                                                unit: "s")
+      #
+      # @param name [String] the name of the up_down_counter
+      # @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 up_down_counter, it will be stored in instrument_registry
       def create_up_down_counter(name, unit: nil, description: nil)
         create_instrument(:up_down_counter, name, unit, description, nil) { UP_DOWN_COUNTER }
       end
 
+      # ObservableCounter is an asynchronous Instrument which reports monotonically
+      # increasing value(s) when the instrument is being observed.
+      #
+      # With this api call:
+      #
+      #   pf_callback = -> { # collect metrics here }
+      #   meter.create_observable_counter("PF",
+      #                                    pf_callback,
+      #                                    description: "process page faults",
+      #                                    unit: 'ms')
+      #
+      #
+      # @param name [String] the name of the observable_counter
+      # @param callback [Proc] the callback function that used to collect metrics
+      # @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 observable_counter, it will be stored in instrument_registry
       def create_observable_counter(name, callback:, unit: nil, description: nil)
         create_instrument(:observable_counter, name, unit, description, callback) { OBSERVABLE_COUNTER }
       end
 
+      # ObservableGauge is an asynchronous Instrument which reports non-additive value(s)
+      # (e.g. the room temperature - it makes no sense to report the temperature value
+      # from multiple rooms and sum them up) when the instrument is being observed.
+      #
+      # With this api call:
+      #
+      #   pf_callback = -> { # collect metrics here }
+      #   meter.create_observable_counter("cpu.frequency",
+      #                                    pf_callback,
+      #                                    description: "the real-time CPU clock speed",
+      #                                    unit: 'ms')
+      #
+      #
+      # @param name [String] the name of the observable_gauge
+      # @param callback [Proc] the callback function that used to collect metrics
+      # @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 observable_gauge, it will be stored in instrument_registry
       def create_observable_gauge(name, callback:, unit: nil, description: nil)
         create_instrument(:observable_gauge, name, unit, description, callback) { OBSERVABLE_GAUGE }
       end
 
+      # ObservableUpDownCounter is an asynchronous Instrument which reports additive value(s)
+      # (e.g. the process heap size - it makes sense to report the heap size from multiple processes
+      # and sum them up, so we get the total heap usage) when the instrument is being observed.
+      #
+      # With this api call:
+      #
+      #   pf_callback = -> { # collect metrics here }
+      #   meter.create_observable_up_down_counter("process.workingset",
+      #                                            pf_callback,
+      #                                            description: "process working set",
+      #                                            unit: 'KB')
+      #
+      #
+      # @param name [String] the name of the observable_up_down_counter
+      # @param callback [Proc] the callback function that used to collect metrics
+      # @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 observable_up_down_counter, it will be stored in instrument_registry
       def create_observable_up_down_counter(name, callback:, unit: nil, description: nil)
         create_instrument(:observable_up_down_counter, name, unit, description, callback) { OBSERVABLE_UP_DOWN_COUNTER }
       end