Track method call times/counts as a single metric

Previously we'd create a separate Metric instance for every method call
that would exceed the method call threshold. This is problematic because
it doesn't provide us with information to accurately get the _total_
execution time of a particular method. For example, if the method
"Foo#bar" was called 4 times with a runtime of ~10 milliseconds we'd end
up with 4 different Metric instances. If we were to then get the
average/95th percentile/etc of the timings this would be roughly 10
milliseconds. However, the _actual_ total time spent in this method
would be around 40 milliseconds.

To solve this problem we now create a single Metric instance per method.
This Metric instance contains the _total_ real/CPU time and the call
count for every instrumented method.

1 files changed, 16 insertions, 0 deletions

diff --git a/spec/lib/gitlab/metrics/transaction_spec.rb b/spec/lib/gitlab/metrics/transaction_spec.rb
index 1d5a51a157e..3b1c67a2147 100644
--- a/spec/lib/gitlab/metrics/transaction_spec.rb
+++ b/spec/lib/gitlab/metrics/transaction_spec.rb
@@ -46,6 +46,22 @@ describe Gitlab::Metrics::Transaction do
     end
   end
 
+  describe '#measure_method' do
+    it 'adds a new method if it does not exist already' do
+      transaction.measure_method('Foo#bar') { 'foo' }
+
+      expect(transaction.methods['Foo#bar']).
+        to be_an_instance_of(Gitlab::Metrics::MethodCall)
+    end
+
+    it 'adds timings to an existing method call' do
+      transaction.measure_method('Foo#bar') { 'foo' }
+      transaction.measure_method('Foo#bar') { 'foo' }
+
+      expect(transaction.methods['Foo#bar'].call_count).to eq(2)
+    end
+  end
+
   describe '#increment' do
     it 'increments a counter' do
       transaction.increment(:time, 1)