Fix rubocop warnings

lib/gitlab/metrics/method_call.rb

@@ -2,7 +2,7 @@ module Gitlab
   module Metrics
     # Class for tracking timing information about method calls
     class MethodCall
-      BASE_LABELS = { module: nil, method: nil }
+      BASE_LABELS = { module: nil, method: nil }.freeze
       attr_reader :real_time, :cpu_time, :call_count
 
       def self.call_real_duration_histogram

lib/gitlab/metrics/samplers/base_sampler.rb

@@ -62,4 +62,3 @@ module Gitlab
     end
   end
 end
-

lib/gitlab/metrics/samplers/influx_sampler.rb

@@ -100,4 +100,4 @@ module Gitlab
       end
     end
   end
-end
\ No newline at end of file
+end

lib/gitlab/metrics/samplers/unicorn_sampler.rb

@@ -47,4 +47,4 @@ module Gitlab
       end
     end
   end
-end
\ No newline at end of file
+end