Ability to increment custom transaction values

This will be used to store/increment the total query/view rendering
timings on a per transaction basis. This in turn can greatly reduce the
amount of metrics stored.

lib/gitlab/metrics/transaction.rb

@@ -19,6 +19,7 @@ module Gitlab
         @started_at  = nil
         @finished_at = nil
 
+        @values = Hash.new(0)
         @tags   = {}
       end
 
@@ -44,6 +45,10 @@ module Gitlab
         @metrics << Metric.new(series, values, tags)
       end
 
+      def increment(name, value)
+        @values[name] += value
+      end
+
       def add_tag(key, value)
         @tags[key] = value
       end
@@ -54,7 +59,13 @@ module Gitlab
       end
 
       def track_self
-        add_metric(@series, { duration: duration }, @tags)
+        values = { duration: duration }
+
+        @values.each do |name, value|
+          values[name] = value
+        end
+
+        add_metric(@series, values, @tags)
       end
 
       def submit

spec/lib/gitlab/metrics/transaction_spec.rb

@@ -38,6 +38,18 @@ describe Gitlab::Metrics::Transaction do
     end
   end
 
+  describe '#increment' do
+    it 'increments a counter' do
+      transaction.increment(:time, 1)
+      transaction.increment(:time, 2)
+
+      expect(transaction).to receive(:add_metric).
+        with('rspec', { duration: 0.0, time: 3 }, {})
+
+      transaction.track_self
+    end
+  end
+
   describe '#add_tag' do
     it 'adds a tag' do
       transaction.add_tag(:foo, 'bar')