metrics: add getter method automatically (#4978)

**Which issue(s) this PR fixes**: 
Fixes #

**What this PR does / why we need it**: 
This PR aim to refactor preparing the getter method of metrics.
In addition, the methods are defined automatically, it prevents we
forget to add getter.

**Docs Changes**:
TODO: Seems we need to create a doc for plugin helper metrics.

**Release Note**: 
Same as the title.

---------

Signed-off-by: Shizuo Fujita <[email protected]>

Author: Watson1978

lib/fluent/plugin/bare_output.rb

@@ -40,22 +40,6 @@ def process(tag, es)
         raise NotImplementedError, "BUG: output plugins MUST implement this method"
       end
 
-      def num_errors
-        @num_errors_metrics.get
-      end
-
-      def emit_count
-        @emit_count_metrics.get
-      end
-
-      def emit_size
-        @emit_size_metrics.get
-      end
-
-      def emit_records
-        @emit_records_metrics.get
-      end
-
       def initialize
         super
         @counter_mutex = Mutex.new

lib/fluent/plugin/buffer.rb

@@ -196,6 +196,8 @@ def initialize
         @mutex = Mutex.new
       end
 
+      # The metrics_create method defines getter methods named stage_byte_size and queue_byte_size.
+      # For compatibility, stage_size, stage_size=, queue_size, and queue_size= are still available.
       def stage_size
         @stage_size_metrics.get
       end

lib/fluent/plugin/filter.rb

@@ -41,14 +41,6 @@ def initialize
         @enable_size_metrics = false
       end
 
-      def emit_records
-        @emit_records_metrics.get
-      end
-
-      def emit_size
-        @emit_size_metrics.get
-      end
-
       def configure(conf)
         super
 

lib/fluent/plugin/input.rb

@@ -37,14 +37,6 @@ def initialize
         @enable_size_metrics = false
       end
 
-      def emit_records
-        @emit_records_metrics.get
-      end
-
-      def emit_size
-        @emit_size_metrics.get
-      end
-
       def configure(conf)
         super
 

lib/fluent/plugin/multi_output.rb

@@ -56,22 +56,6 @@ def initialize
         @enable_size_metrics = false
       end
 
-      def num_errors
-        @num_errors_metrics.get
-      end
-
-      def emit_count
-        @emit_count_metrics.get
-      end
-
-      def emit_size
-        @emit_size_metrics.get
-      end
-
-      def emit_records
-        @emit_records_metrics.get
-      end
-
       def statistics
         stats = {
           'num_errors' => @num_errors_metrics.get,

lib/fluent/plugin/output.rb

@@ -171,30 +171,6 @@ def expired?
       # output_enqueue_thread_waiting: for test of output.rb itself
       attr_accessor :retry_for_error_chunk # if true, error flush will be retried even if under_plugin_development is true
 
-      def num_errors
-        @num_errors_metrics.get
-      end
-
-      def emit_count
-        @emit_count_metrics.get
-      end
-
-      def emit_size
-        @emit_size_metrics.get
-      end
-
-      def emit_records
-        @emit_records_metrics.get
-      end
-
-      def write_count
-        @write_count_metrics.get
-      end
-
-      def rollback_count
-        @rollback_count_metrics.get
-      end
-
       def initialize
         super
         @counter_mutex = Mutex.new

lib/fluent/plugin_helper/metrics.rb

@@ -72,6 +72,13 @@ def metrics_create(namespace: "fluentd", subsystem: "metrics", name:, help_text:
 
         @_metrics["#{@plugin_type_or_id}_#{namespace}_#{subsystem}_#{name}"] = metrics
 
+        # define the getter method for the calling instance.
+        singleton_class.module_eval do
+          unless method_defined?(name)
+            define_method(name) { metrics.get }
+          end
+        end
+
         metrics
       end
 

test/plugin_helper/test_metrics.rb

@@ -134,4 +134,18 @@ def configure(conf)
     assert i2.terminated?
     assert i3.terminated?
   end
+
+  test 'can create getter method by metrics name' do
+    @d = d = Dummy.new
+
+    assert_raise(NoMethodError) do
+      d.foobarbaz
+    end
+
+    metrics = d.metrics_create(namespace: "fluentd_test", subsystem: "unit-test", name: "foobarbaz", help_text: "metrics testing")
+    metrics.inc
+
+    assert_equal(1, d.foobarbaz)
+    assert_equal(1, metrics.get)
+  end
 end