Use class variables for metrics provider cache

lib/gitlab/metrics/concern.rb

@@ -7,12 +7,12 @@ module Gitlab
 
       included do
         @@_metric_provider_mutex ||= Mutex.new
-        @_metrics_provider_cache = {}
+        @@_metrics_provider_cache = {}
       end
 
       class_methods do
         def reload_metric!(name)
-          @_metrics_provider_cache.delete(name)
+          @@_metrics_provider_cache.delete(name)
         end
 
         private
@@ -23,12 +23,12 @@ module Gitlab
           end
 
           define_singleton_method(name) do
-            @_metrics_provider_cache&.[](name) || init_metric(type, name, opts, &block)
+            @@_metrics_provider_cache[name] || init_metric(type, name, opts, &block)
           end
         end
 
         def fetch_metric(type, name, opts = {}, &block)
-          @_metrics_provider_cache&.[](name) || init_metric(type, name, opts, &block)
+          @@_metrics_provider_cache[name] || init_metric(type, name, opts, &block)
         end
 
         def init_metric(type, name, opts = {}, &block)
@@ -44,8 +44,7 @@ module Gitlab
 
         def synchronized_cache_fill(key)
           @@_metric_provider_mutex.synchronize do
-            @_metrics_provider_cache ||= {}
-            @_metrics_provider_cache[key] ||= yield
+            @@_metrics_provider_cache[key] ||= yield
           end
         end