Convert InfluxDB to concern. Fix uninitialized metrics when metrics code is inherited.

lib/gitlab/metrics.rb

 module Gitlab
   module Metrics
-    extend Gitlab::Metrics::InfluxDb
+    include Gitlab::Metrics::InfluxDb
     include Gitlab::Metrics::Prometheus
 
     def self.enabled?

lib/gitlab/metrics/concern.rb

@@ -6,7 +6,7 @@ module Gitlab
       extend ActiveSupport::Concern
 
       included do
-        @_metric_provider_mutex = Mutex.new
+        @@_metric_provider_mutex = Mutex.new
         @_metrics_provider_cache = {}
       end
 
@@ -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)
@@ -43,7 +43,8 @@ module Gitlab
         end
 
         def synchronized_cache_fill(key)
-          @_metric_provider_mutex.synchronize do
+          @@_metric_provider_mutex.synchronize do
+            @_metrics_provider_cache ||= {}
             @_metrics_provider_cache[key] ||= yield
           end
         end

lib/gitlab/metrics/prometheus.rb

@@ -4,13 +4,12 @@ module Gitlab
   module Metrics
     module Prometheus
       extend ActiveSupport::Concern
-      include Gitlab::Metrics::Concern
-      include Gitlab::CurrentSettings
 
       REGISTRY_MUTEX = Mutex.new
       PROVIDER_MUTEX = Mutex.new
 
       class_methods do
+        include Gitlab::CurrentSettings
         include Gitlab::Utils::StrongMemoize
 
         def metrics_folder_present?

spec/lib/gitlab/metrics/subscribers/rails_cache_spec.rb

@@ -144,7 +144,10 @@ describe Gitlab::Metrics::Subscribers::RailsCache do
     end
 
     context 'with a transaction' do
+      let(:metric_cache_misses_total) { double('metric_cache_misses_total', increment: nil) }
+
       before do
+        allow(subscriber).to receive(:metric_cache_misses_total).and_return(metric_cache_misses_total)
         allow(subscriber).to receive(:current_transaction)
                                .and_return(transaction)
       end
@@ -157,9 +160,9 @@ describe Gitlab::Metrics::Subscribers::RailsCache do
       end
 
       it 'increments the cache_read_miss total' do
-        expect(subscriber.send(:metric_cache_misses_total)).to receive(:increment).with({})
-
         subscriber.cache_generate(event)
+
+        expect(metric_cache_misses_total).to have_received(:increment).with({})
       end
     end
   end