Use string evaluation for method instrumentation

This is faster than using define_method since we don't have to keep
block bindings around.

lib/gitlab/metrics/instrumentation.rb

@@ -31,16 +31,18 @@ module Gitlab
         alias_name = "_original_#{name}"
         target     = type == :instance ? mod : mod.singleton_class
 
-        target.class_eval do
-          alias_method(alias_name, name)
+        target.class_eval <<-EOF, __FILE__, __LINE__ + 1
+          alias_method :#{alias_name}, :#{name}
 
-          define_method(name) do |*args, &block|
-            ActiveSupport::Notifications.
-              instrument("#{type}_method.method_call", module: mod, name: name) do
-                __send__(alias_name, *args, &block)
+          def #{name}(*args, &block)
+            ActiveSupport::Notifications
+              .instrument("#{type}_method.method_call",
+                          module: #{mod.name.inspect},
+                          name: #{name.inspect}) do
+                #{alias_name}(*args, &block)
               end
           end
-        end
+        EOF
       end
     end
   end

lib/gitlab/metrics/subscribers/method_call.rb

@@ -10,7 +10,7 @@ module Gitlab
         def instance_method(event)
           return unless current_transaction
 
-          label = "#{event.payload[:module].name}##{event.payload[:name]}"
+          label = "#{event.payload[:module]}##{event.payload[:name]}"
 
           add_metric(label, event.duration)
         end
@@ -18,7 +18,7 @@ module Gitlab
         def class_method(event)
           return unless current_transaction
 
-          label = "#{event.payload[:module].name}.#{event.payload[:name]}"
+          label = "#{event.payload[:module]}.#{event.payload[:name]}"
 
           add_metric(label, event.duration)
         end

spec/lib/gitlab/metrics/instrumentation_spec.rb

@@ -11,6 +11,8 @@ describe Gitlab::Metrics::Instrumentation do
         text
       end
     end
+
+    allow(@dummy).to receive(:name).and_return('Dummy')
   end
 
   describe '.instrument_method' do
@@ -31,7 +33,7 @@ describe Gitlab::Metrics::Instrumentation do
 
       it 'fires an ActiveSupport notification upon calling the method' do
         expect(ActiveSupport::Notifications).to receive(:instrument).
-          with('class_method.method_call', module: @dummy, name: :foo)
+          with('class_method.method_call', module: 'Dummy', name: :foo)
 
         @dummy.foo
       end
@@ -69,7 +71,7 @@ describe Gitlab::Metrics::Instrumentation do
 
       it 'fires an ActiveSupport notification upon calling the method' do
         expect(ActiveSupport::Notifications).to receive(:instrument).
-          with('instance_method.method_call', module: @dummy, name: :bar)
+          with('instance_method.method_call', module: 'Dummy', name: :bar)
 
         @dummy.new.bar
       end

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

@@ -6,8 +6,7 @@ describe Gitlab::Metrics::Subscribers::MethodCall do
   let(:subscriber) { described_class.new }
 
   let(:event) do
-    double(:event, duration: 0.2,
-                   payload:  { module: double(:mod, name: 'Foo'), name: :foo })
+    double(:event, duration: 0.2, payload: { module: 'Foo', name: :foo })
   end
 
   before do