Send number of allocations in span as metric

I recently noticed that while we usually collect the number of memory
allocations on a given span, we send it as an "allocations" field to
the agent and this data never seems to make it to the backend.

Instead, this refactor reports it as a span metric, which makes
it available to customers.

lib/ddtrace/span.rb

@@ -28,7 +28,6 @@ class Span
     EXTERNAL_MAX_ID = 1 << 64
 
     attr_accessor \
-      :allocations,
       :end_time,
       :id,
       :meta,
@@ -62,7 +61,6 @@ class Span
     # TODO: Remove span_type
     def initialize(
       name,
-      allocations: 0,
       duration: nil,
       end_time: nil,
       id: nil,
@@ -90,8 +88,6 @@ def initialize(
       @metrics = metrics || {}
       @status = status || 0
 
-      @allocations = allocations || 0
-
       # start_time and end_time track wall clock. In Ruby, wall clock
       # has less accuracy than monotonic clock, so if possible we look to only use wall clock
       # to measure duration when a time is supplied by the user, or if monotonic clock
@@ -141,7 +137,6 @@ def to_s
     # isn't handled by this method.
     def to_hash
       h = {
-        allocations: @allocations,
         error: @status,
         meta: @meta,
         metrics: @metrics,
@@ -179,7 +174,6 @@ def pretty_print(q)
         q.text "Start: #{start_time}\n"
         q.text "End: #{end_time}\n"
         q.text "Duration: #{duration.to_f}\n"
-        q.text "Allocations: #{allocations}\n"
         q.group(2, 'Tags: [', "]\n") do
           q.breakable
           q.seplist @meta.each do |key, value|

lib/ddtrace/span_operation.rb

@@ -182,6 +182,8 @@ def stop(stop_time = nil)
 
       @allocation_count_stop = now_allocations
 
+      set_metric('allocations', allocations)
+
       now = Utils::Time.now.utc
 
       # Provide a default start_time if unset.
@@ -261,7 +263,6 @@ def to_s
     # Return the hash representation of the current span.
     def to_hash
       h = {
-        allocations: allocations,
         error: @status,
         id: @id,
         meta: meta,
@@ -299,7 +300,6 @@ def pretty_print(q)
         q.text "Start: #{start_time}\n"
         q.text "End: #{end_time}\n"
         q.text "Duration: #{duration.to_f if stopped?}\n"
-        q.text "Allocations: #{allocations}\n"
         q.group(2, 'Tags: [', "]\n") do
           q.breakable
           q.seplist meta.each do |key, value|
@@ -408,7 +408,6 @@ def message
     def build_span
       Span.new(
         @name && @name.dup,
-        allocations: allocations,
         duration: duration,
         end_time: @end_time,
         id: @id,
@@ -462,7 +461,7 @@ def duration_nano
       (duration * 1e9).to_i
     end
 
-    if defined?(JRUBY_VERSION) || Gem::Version.new(RUBY_VERSION) < Gem::Version.new(VERSION::MINIMUM_RUBY_VERSION)
+    if defined?(JRUBY_VERSION)
       def now_allocations
         0
       end

lib/ddtrace/transport/serializable_trace.rb

@@ -53,16 +53,18 @@ def initialize(span)
       def to_msgpack(packer = nil)
         packer ||= MessagePack::Packer.new
 
+        number_of_elements_to_write = 10
+
         if span.stopped?
-          packer.write_map_header(13) # Set header with how many elements in the map
+          packer.write_map_header(number_of_elements_to_write + 2) # Set header with how many elements in the map
 
           packer.write('start')
           packer.write(time_nano(span.start_time))
 
           packer.write('duration')
           packer.write(duration_nano(span.duration))
         else
-          packer.write_map_header(11) # Set header with how many elements in the map
+          packer.write_map_header(number_of_elements_to_write) # Set header with how many elements in the map
         end
 
         # DEV: We use strings as keys here, instead of symbols, as
@@ -86,8 +88,6 @@ def to_msgpack(packer = nil)
         packer.write(span.meta)
         packer.write('metrics')
         packer.write(span.metrics)
-        packer.write('allocations')
-        packer.write(span.allocations)
         packer.write('error')
         packer.write(span.status)
         packer

spec/ddtrace/span_operation_spec.rb

@@ -511,6 +511,14 @@
       context 'then #stop' do
         before { start }
         it { expect { span_op.stop }.to change { span_op.duration }.from(nil).to(kind_of(Float)) }
+
+        it 'sets the measured number of allocations as a metric' do
+          skip 'Measuring allocations is not supported on JRuby' if PlatformHelpers.jruby?
+
+          span_op.stop
+
+          expect(span_op.to_hash[:metrics]).to include('allocations' => a_value > 0)
+        end
       end
 
       context 'and callbacks have been configured' do
@@ -879,14 +887,17 @@
   end
 
   describe '#allocations' do
+    before do
+      skip 'Measuring allocations is not supported on JRuby' if PlatformHelpers.jruby?
+    end
+
     subject(:allocations) { span_op.allocations }
 
     it { is_expected.to be 0 }
 
     context 'when span measures an operation' do
       before do
-        skip 'Test unstable; improve stability before re-enabling.'
-        span_op.measure {}
+        span_op.measure { Object.new }
       end
 
       it { is_expected.to be > 0 }
@@ -895,7 +906,7 @@
         let(:span_op_two) { described_class.new('span_op_two') }
 
         before do
-          span_op_two.measure { Object.new }
+          span_op_two.measure { 10_000.times { Object.new } }
         end
 
         it { is_expected.to be < span_op_two.allocations }

spec/ddtrace/span_spec.rb

@@ -213,7 +213,6 @@
         type: nil,
         meta: {},
         metrics: {},
-        allocations: 0,
         error: 0
       )
     end