Reduce instrumentation overhead

This reduces the overhead of the method instrumentation code primarily
by reducing the number of method calls. There are also some other small
optimisations such as not casting timing values to Floats (there's no
particular need for this), using Symbols for method call metric names,
and reducing the number of Hash lookups for instrumented methods.

The exact impact depends on the code being executed. For example, for a
method that's only called once the difference won't be very noticeable.
However, for methods that are called many times the difference can be
more significant.

For example, the loading time of a large commit
(nrclark/dummy_project@81ebdea5df2fb42e59257cb3eaad671a5c53ca36)
was reduced from around 19 seconds to around 15 seconds using these
changes.

1 files changed, 6 insertions, 21 deletions

diff --git a/lib/gitlab/metrics/transaction.rb b/lib/gitlab/metrics/transaction.rb
index bded245da43..968f3218950 100644
--- a/lib/gitlab/metrics/transaction.rb
+++ b/lib/gitlab/metrics/transaction.rb
@@ -52,23 +52,16 @@ module Gitlab
       end
 
       def add_metric(series, values, tags = {})
-        @metrics << Metric.new("#{series_prefix}#{series}", values, tags)
+        @metrics << Metric.new("#{Metrics.series_prefix}#{series}", values, tags)
       end
 
-      # Measures the time it takes to execute a method.
-      #
-      # Multiple calls to the same method add up to the total runtime of the
-      # method.
-      #
-      # name - The full name of the method to measure (e.g. `User#sign_in`).
-      def measure_method(name, &block)
-        unless @methods[name]
-          series = "#{series_prefix}#{Instrumentation::SERIES}"
-
-          @methods[name] = MethodCall.new(name, series)
+      # Returns a MethodCall object for the given name.
+      def method_call_for(name)
+        unless method = @methods[name]
+          @methods[name] = method = MethodCall.new(name, Instrumentation.series)
         end
 
-        @methods[name].measure(&block)
+        method
       end
 
       def increment(name, value)
@@ -115,14 +108,6 @@ module Gitlab
 
         Metrics.submit_metrics(submit_hashes)
       end
-
-      def sidekiq?
-        Sidekiq.server?
-      end
-
-      def series_prefix
-        sidekiq? ? 'sidekiq_' : 'rails_'
-      end
     end
   end
 end