Removed tracking of total method execution times

Because method call timings are inclusive (that is, they include the
time of any sub method calls) this would lead to the total method
execution time often being far greater than the total transaction time.
Because this is incredibly confusing it's best to simply _not_ track the
total method execution time, after all it's not that useful to begin
with.

Fixes gitlab-org/gitlab-ce#17239

CHANGELOG

@@ -43,6 +43,7 @@ v 8.8.0 (unreleased)
   - Fix unintentional filtering bug in issues sorted by milestone due (Takuya Noguchi)
   - Fix adding a todo for private group members (Ahmad Sherif)
   - Bump ace-rails-ap gem version from 2.0.1 to 4.0.2 which upgrades Ace Editor from 1.1.2 to 1.2.3
+  - Total method execution timings are no longer tracked
 
 v 8.7.5
   - Fix relative links in wiki pages. !4050

lib/gitlab/metrics/instrumentation.rb

@@ -154,8 +154,6 @@ module Gitlab
               duration = (Time.now - start) * 1000.0
 
               if duration >= Gitlab::Metrics.method_call_threshold
-                trans.increment(:method_duration, duration)
-
                 trans.add_metric(Gitlab::Metrics::Instrumentation::SERIES,
                                  { duration: duration },
                                  method: #{label.inspect})

spec/lib/gitlab/metrics/instrumentation_spec.rb

@@ -56,9 +56,6 @@ describe Gitlab::Metrics::Instrumentation do
         allow(described_class).to receive(:transaction).
           and_return(transaction)
 
-        expect(transaction).to receive(:increment).
-          with(:method_duration, a_kind_of(Numeric))
-
         expect(transaction).to receive(:add_metric).
           with(described_class::SERIES, an_instance_of(Hash),
                method: 'Dummy.foo')
@@ -139,9 +136,6 @@ describe Gitlab::Metrics::Instrumentation do
         allow(described_class).to receive(:transaction).
           and_return(transaction)
 
-        expect(transaction).to receive(:increment).
-          with(:method_duration, a_kind_of(Numeric))
-
         expect(transaction).to receive(:add_metric).
           with(described_class::SERIES, an_instance_of(Hash),
                method: 'Dummy#bar')