Fix Gitlab::Metrics metaprogramming magic

On initial startup with no rows in the application_settings table, the
metaprogramming call to `if enabled?` attempts to create a row. This triggers
the HTML caching path, which attempts to store metrics.

At this point, not all the methods in `Gitlab::Metrics` have been defined! Move
`current_transaction` to be defined before running the metaprogramming, to
avoid a confusing NoMethodError

lib/gitlab/metrics.rb

@@ -138,6 +138,11 @@ module Gitlab
       @series_prefix ||= Sidekiq.server? ? 'sidekiq_' : 'rails_'
     end
 
+    # Allow access from other metrics related middlewares
+    def self.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 enabled?
@@ -149,10 +154,5 @@ module Gitlab
           new(udp: { host: host, port: port })
       end
     end
-
-    # Allow access from other metrics related middlewares
-    def self.current_transaction
-      Transaction.current
-    end
   end
 end