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, 2 insertions, 17 deletions

diff --git a/doc/development/instrumentation.md b/doc/development/instrumentation.md
index 6cd9b274d11..c2272ab0a2b 100644
--- a/doc/development/instrumentation.md
+++ b/doc/development/instrumentation.md
@@ -94,23 +94,8 @@ Visibility: public
 Number of lines: 21
 
 def #{name}(#{args_signature})
-  trans = Gitlab::Metrics::Instrumentation.transaction
-
-  if trans
-    start     = Time.now
-    cpu_start = Gitlab::Metrics::System.cpu_time
-    retval    = super
-    duration  = (Time.now - start) * 1000.0
-
-    if duration >= Gitlab::Metrics.method_call_threshold
-      cpu_duration = Gitlab::Metrics::System.cpu_time - cpu_start
-
-      trans.add_metric(Gitlab::Metrics::Instrumentation::SERIES,
-                       { duration: duration, cpu_duration: cpu_duration },
-                       method: #{label.inspect})
-    end
-
-    retval
+  if trans = Gitlab::Metrics::Instrumentation.transaction
+    trans.measure_method(#{label.inspect}) { super }
   else
     super
   end