Add missing mutex guard to method call metrics

lib/gitlab/metrics/method_call.rb

@@ -2,10 +2,14 @@ module Gitlab
   module Metrics
     # Class for tracking timing information about method calls
     class MethodCall
+      MUTEX = Mutex.new
       BASE_LABELS = { module: nil, method: nil }.freeze
       attr_reader :real_time, :cpu_time, :call_count, :labels
 
       def self.call_real_duration_histogram
+        return @call_real_duration_histogram if @call_real_duration_histogram
+
+        MUTEX.synchronize do
           @call_real_duration_histogram ||= Gitlab::Metrics.histogram(
             :gitlab_method_call_real_duration_seconds,
             'Method calls real duration',
@@ -13,8 +17,12 @@ module Gitlab
             [0.1, 0.2, 0.5, 1, 2, 5, 10]
           )
         end
+      end
 
       def self.call_cpu_duration_histogram
+        return @call_cpu_duration_histogram if @call_cpu_duration_histogram
+
+        MUTEX.synchronize do
           @call_duration_histogram ||= Gitlab::Metrics.histogram(
             :gitlab_method_call_cpu_duration_seconds,
             'Method calls cpu duration',
@@ -22,6 +30,7 @@ module Gitlab
             [0.1, 0.2, 0.5, 1, 2, 5, 10]
           )
         end
+      end
 
       # name - The full name of the method (including namespace) such as
       #        `User#sign_in`.