NullMetric as a singleton

lib/gitlab/metrics/concern.rb

@@ -34,7 +34,7 @@ module Gitlab
           options.evaluate(&block)
 
           if disabled_by_feature(options)
-            synchronized_cache_fill(name) { NullMetric.new }
+            synchronized_cache_fill(name) { NullMetric.instance }
           else
             synchronized_cache_fill(name) { build_metric!(type, name, options) }
           end

lib/gitlab/metrics/null_metric.rb

@@ -2,6 +2,8 @@ module Gitlab
   module Metrics
     # Mocks ::Prometheus::Client::Metric and all derived metrics
     class NullMetric
+      include Singleton
+
       def method_missing(name, *args, &block)
         nil
       end

lib/gitlab/metrics/prometheus.rb

@@ -64,7 +64,7 @@ module Gitlab
         if prometheus_metrics_enabled?
           registry.get(name)
         else
-          NullMetric.new
+          NullMetric.instance
         end
       end
 

spec/lib/gitlab/metrics/concern_spec.rb

@@ -53,7 +53,9 @@ describe Gitlab::Metrics::Concern do
 
     describe "#fetch_#{metric_type}" do
       let(:fetch_method) { "fetch_#{metric_type}".to_sym }
-      let(:null_metric) { Gitlab::Metrics::NullMetric.new }
+      let(:_metric_type) { metric_type }
+
+      let(:null_metric) { Gitlab::Metrics::NullMetric.instance }
 
       context "when #{metric_type} is not cached" do
         it 'initializes counter metric' do