Improved last_relative_application_frame timings

The previous setup wasn't exactly fast, resulting in instrumented method
calls taking about 600 times longer than non instrumented calls
(including any ActiveSupport code involved). With this commit this
slowdown has been reduced to around 185 times.

lib/gitlab/metrics.rb

 module Gitlab
   module Metrics
+    RAILS_ROOT   = Rails.root.to_s
+    METRICS_ROOT = Rails.root.join('lib', 'gitlab', 'metrics').to_s
+    PATH_REGEX   = /^#{RAILS_ROOT}\/?/
+
     def self.pool_size
       Settings.metrics['pool_size'] || 16
     end
@@ -20,16 +24,15 @@ module Gitlab
       @hostname
     end
 
+    # Returns a relative path and line number based on the last application call
+    # frame.
     def self.last_relative_application_frame
-      root    = Rails.root.to_s
-      metrics = Rails.root.join('lib', 'gitlab', 'metrics').to_s
-
       frame = caller_locations.find do |l|
-        l.path.start_with?(root) && !l.path.start_with?(metrics)
+        l.path.start_with?(RAILS_ROOT) && !l.path.start_with?(METRICS_ROOT)
       end
 
       if frame
-        return frame.path.gsub(/^#{Rails.root.to_s}\/?/, ''), frame.lineno
+        return frame.path.sub(PATH_REGEX, ''), frame.lineno
       else
         return nil, nil
       end