Only track method calls above a certain threshold

This ensures we don't end up wasting resources by tracking method calls
that only take a few microseconds. By default the threshold is 10
milliseconds but this can be changed using the gitlab.yml configuration
file.

config/gitlab.yml.example

@@ -432,6 +432,9 @@ production: &base
     # pool_size: 16
     # The timeout of a connection in seconds.
     # timeout: 10
+    # The minimum amount of milliseconds a method call has to take before it's
+    # tracked. Defaults to 10.
+    # method_call_threshold: 10
 
 development:
   <<: *base

lib/gitlab/metrics.rb

@@ -16,6 +16,10 @@ module Gitlab
       !!Settings.metrics['enabled']
     end
 
+    def self.method_call_threshold
+      Settings.metrics['method_call_threshold'] || 10
+    end
+
     def self.pool
       @pool
     end

lib/gitlab/metrics/instrumentation.rb

@@ -99,9 +99,11 @@ module Gitlab
               retval   = __send__(#{alias_name.inspect}, *args, &block)
               duration = (Time.now - start) * 1000.0
 
-              trans.add_metric(Gitlab::Metrics::Instrumentation::SERIES,
-                               { duration: duration },
-                               method: #{label.inspect})
+              if duration >= Gitlab::Metrics.method_call_threshold
+                trans.add_metric(Gitlab::Metrics::Instrumentation::SERIES,
+                                 { duration: duration },
+                                 method: #{label.inspect})
+              end
 
               retval
             else

spec/lib/gitlab/metrics/instrumentation_spec.rb

@@ -42,6 +42,9 @@ describe Gitlab::Metrics::Instrumentation do
       end
 
       it 'tracks the call duration upon calling the method' do
+        allow(Gitlab::Metrics).to receive(:method_call_threshold).
+          and_return(0)
+
         allow(described_class).to receive(:transaction).
           and_return(transaction)
 
@@ -51,6 +54,15 @@ describe Gitlab::Metrics::Instrumentation do
 
         @dummy.foo
       end
+
+      it 'does not track method calls below a given duration threshold' do
+        allow(Gitlab::Metrics).to receive(:method_call_threshold).
+          and_return(100)
+
+        expect(transaction).to_not receive(:add_metric)
+
+        @dummy.foo
+      end
     end
 
     describe 'with metrics disabled' do
@@ -84,6 +96,9 @@ describe Gitlab::Metrics::Instrumentation do
       end
 
       it 'tracks the call duration upon calling the method' do
+        allow(Gitlab::Metrics).to receive(:method_call_threshold).
+          and_return(0)
+
         allow(described_class).to receive(:transaction).
           and_return(transaction)
 
@@ -93,6 +108,15 @@ describe Gitlab::Metrics::Instrumentation do
 
         @dummy.new.bar
       end
+
+      it 'does not track method calls below a given duration threshold' do
+        allow(Gitlab::Metrics).to receive(:method_call_threshold).
+          and_return(100)
+
+        expect(transaction).to_not receive(:add_metric)
+
+        @dummy.new.bar
+      end
     end
 
     describe 'with metrics disabled' do