Fix circular dependency condition with `current_application_settings`

`current_application_settings` used by `influx_metrics_enabled`
executed a markdown parsing code that was measured using `Gitlab::Metrics.measure`

But since the Gitlab::Metrics::InfluxDb was not yet build so Gitlab::Metrics
did not yet have `measure` method. Causing the NoMethodError.

However If run was successful at least once then result was cached in a file and this code never executed again.
Which caused this issue to only show up in CI preparation step.

lib/gitlab/metrics/influx_db.rb

 module Gitlab
   module Metrics
     module InfluxDb
-      include Gitlab::CurrentSettings
+      extend Gitlab::CurrentSettings
+      extend self
+
+      MUTEX = Mutex.new
+      private_constant :MUTEX
 
       def influx_metrics_enabled?
         settings[:enabled] || false
@@ -35,10 +39,6 @@ module Gitlab
         @method_call_threshold ||= settings[:method_call_threshold]
       end
 
-      def pool
-        @pool
-      end
-
       def submit_metrics(metrics)
         prepared = prepare_metrics(metrics)
 
@@ -143,21 +143,28 @@ module Gitlab
       end
 
       # Allow access from other metrics related middlewares
-      def current_transaction 
+      def current_transaction
         Transaction.current
       end
 
       # When enabled this should be set before being used as the usual pattern
       # "@foo ||= bar" is _not_ thread-safe.
-      if influx_metrics_enabled?
-        @pool = ConnectionPool.new(size: settings[:pool_size], timeout: settings[:timeout]) do
-          host = settings[:host]
-          port = settings[:port]
-
-          InfluxDB::Client.
-            new(udp: { host: host, port: port })
+      def pool
+        if influx_metrics_enabled?
+          if @pool.nil?
+            MUTEX.synchronize do
+              @pool ||= ConnectionPool.new(size: settings[:pool_size], timeout: settings[:timeout]) do
+                host = settings[:host]
+                port = settings[:port]
+
+                InfluxDB::Client.
+                  new(udp: { host: host, port: port })
+              end
+            end
+          end
+          @pool
         end
       end
     end
   end
-end
+end
\ No newline at end of file