Use custom code for instrumenting method calls

The use of ActiveSupport would slow down instrumented method calls by
about 180x due to:

1. ActiveSupport itself not being the fastest thing on the planet
2. caller_locations() having quite some overhead

The use of caller_locations() has been removed because it's not _that_
useful since we already know the full namespace of receivers and the
names of the called methods.

The use of ActiveSupport has been replaced with some custom code that's
generated using eval() (which can be quite a bit faster than using
define_method).

This new setup results in instrumented methods only being about 35-40x
slower (compared to non instrumented methods).

Author: Yorick Peterse

config/initializers/metrics.rb

@@ -7,7 +7,6 @@
   # ActiveSupport.
   require 'gitlab/metrics/subscribers/action_view'
   require 'gitlab/metrics/subscribers/active_record'
-  require 'gitlab/metrics/subscribers/method_call'
 
   Gitlab::Application.configure do |config|
     config.middleware.use(Gitlab::Metrics::RackMiddleware)

lib/gitlab/metrics/instrumentation.rb

@@ -9,6 +9,8 @@ module Metrics
     #
     #     Gitlab::Metrics::Instrumentation.instrument_method(User, :by_login)
     module Instrumentation
+      SERIES = 'method_calls'
+
       # Instruments a class method.
       #
       # mod  - The module to instrument as a Module/Class.
@@ -31,19 +33,42 @@ def self.instrument(type, mod, name)
         alias_name = "_original_#{name}"
         target     = type == :instance ? mod : mod.singleton_class
 
+        if type == :instance
+          target = mod
+          label  = "#{mod.name}##{name}"
+        else
+          target = mod.singleton_class
+          label  = "#{mod.name}.#{name}"
+        end
+
         target.class_eval <<-EOF, __FILE__, __LINE__ + 1
           alias_method :#{alias_name}, :#{name}
 
           def #{name}(*args, &block)
-            ActiveSupport::Notifications
-              .instrument("#{type}_method.method_call",
-                          module: #{mod.name.inspect},
-                          name: #{name.inspect}) do
-                #{alias_name}(*args, &block)
-              end
+            trans = Gitlab::Metrics::Instrumentation.transaction
+
+            if trans
+              start    = Time.now
+              retval   = #{alias_name}(*args, &block)
+              duration = (Time.now - start) * 1000.0
+
+              trans.add_metric(Gitlab::Metrics::Instrumentation::SERIES,
+                               { duration: duration },
+                               method: #{label.inspect})
+
+              retval
+            else
+              #{alias_name}(*args, &block)
+            end
           end
         EOF
       end
+
+      # Small layer of indirection to make it easier to stub out the current
+      # transaction.
+      def self.transaction
+        Transaction.current
+      end
     end
   end
 end

lib/gitlab/metrics/subscribers/method_call.rb

@@ -1,6 +1,8 @@
 require 'spec_helper'
 
 describe Gitlab::Metrics::Instrumentation do
+  let(:transaction) { Gitlab::Metrics::Transaction.new }
+
   before do
     @dummy = Class.new do
       def self.foo(text = 'foo')
@@ -31,9 +33,13 @@ def bar(text = 'bar')
         expect(@dummy.foo).to eq('foo')
       end
 
-      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)
+      it 'tracks the call duration upon calling the method' do
+        allow(Gitlab::Metrics::Instrumentation).to receive(:transaction).
+          and_return(transaction)
+
+        expect(transaction).to receive(:add_metric).
+          with(described_class::SERIES, an_instance_of(Hash),
+               method: 'Dummy.foo')
 
         @dummy.foo
       end
@@ -69,9 +75,13 @@ def bar(text = 'bar')
         expect(@dummy.new.bar).to eq('bar')
       end
 
-      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)
+      it 'tracks the call duration upon calling the method' do
+        allow(Gitlab::Metrics::Instrumentation).to receive(:transaction).
+          and_return(transaction)
+
+        expect(transaction).to receive(:add_metric).
+          with(described_class::SERIES, an_instance_of(Hash),
+               method: 'Dummy#bar')
 
         @dummy.new.bar
       end