Track total method call times per transaction

This makes it easier to see where time is spent without having to
aggregate all the individual points in the method_calls series.

lib/gitlab/metrics/instrumentation.rb

@@ -123,6 +123,8 @@ 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

@@ -48,6 +48,9 @@ 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')
@@ -102,6 +105,9 @@ 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')