Avoid cascading locking

lib/gitlab/metrics/concern.rb

@@ -33,19 +33,25 @@ module Gitlab
           options = MetricOptions.new(opts)
           options.evaluate(&block)
 
+          if disabled_by_feature(options)
+            synchronized_cache_fill(name) { NullMetric.new }
+          else
+            synchronized_cache_fill(name) { build_metric!(type, name, options) }
+          end
+        end
+
+        def synchronized_cache_fill(key)
           MUTEX.synchronize do
             @_metrics_provider_cache ||= {}
-            @_metrics_provider_cache[name] ||= build_metric!(type, name, options)
+            @_metrics_provider_cache[key] ||= yield
           end
-
-          @_metrics_provider_cache[name]
         end
 
-        def build_metric!(type, name, options)
-          unless options.with_feature.nil? || Feature.get(options.with_feature).enabled?
-            return NullMetric.new
+        def disabled_by_feature(options)
+          options.with_feature && !Feature.get(options.with_feature).enabled?
         end
 
+        def build_metric!(type, name, options)
           case type
           when :gauge
             Gitlab::Metrics.gauge(name, options.docstring, options.base_labels, options.multiprocess_mode)

spec/lib/gitlab/metrics/concern_spec.rb

@@ -128,5 +128,5 @@ describe Gitlab::Metrics::Concern do
 
   include_examples 'metric', :counter, {}
   include_examples 'metric', :gauge, {}, :all
-  include_examples 'metric', :histogram, {}, [0.005, 0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1, 2.5, 5, 10]
+  include_examples 'metric', :histogram, {}, [0.005, 0.01, 0.1, 1, 10]
 end