add Datadog tracer

Gemfile

@@ -23,6 +23,7 @@ group :development, :test do
   gem "honeybadger"
   gem "coveralls", "~> 0.8.15", require: false
   gem "newrelic_rpm"
+  gem "ddtrace"
   gem "airbrake", "~> 10.0"
   gem "rollbar"
 end

README.md

@@ -188,6 +188,12 @@ This will enable NewRelic custom instrumentation:
 Hutch::Config.set(:tracer, Hutch::Tracers::NewRelic)
 ```
 
+And this will enable Datadog custom instrumentation:
+
+```ruby
+Hutch::Config.set(:tracer, Hutch::Tracers::Datadog)
+```
+
 Batteries included!
 
 ## Running Hutch
@@ -246,7 +252,7 @@ and the consumers are not loaded in development environment you will need to
 trigger the autoloading in an initializer with
 
 ```ruby
-::Zeitwerk::Loader.eager_load_all 
+::Zeitwerk::Loader.eager_load_all
 ```
 
 or with something more specific like

lib/hutch/tracers.rb

@@ -2,5 +2,6 @@ module Hutch
   module Tracers
     autoload :NullTracer, 'hutch/tracers/null_tracer'
     autoload :NewRelic,   'hutch/tracers/newrelic'
+    autoload :Datadog,    'hutch/tracers/datadog'
   end
 end

lib/hutch/tracers/datadog.rb

@@ -0,0 +1,17 @@
+require 'ddtrace'
+
+module Hutch
+  module Tracers
+    class Datadog
+      def initialize(klass)
+        @klass = klass
+      end
+
+      def handle(message)
+        ::Datadog.tracer.trace(@klass.class.name, service: 'hutch', span_type: 'rabbitmq') do
+          @klass.process(message)
+        end
+      end
+    end
+  end
+end

spec/hutch/tracers/datadog_spec.rb

@@ -0,0 +1,44 @@
+require 'spec_helper'
+
+RSpec.describe Hutch::Tracers::Datadog do
+  describe "#handle" do
+    subject(:handle) { tracer.handle(message) }
+
+    let(:tracer) { described_class.new(klass) }
+    let(:klass)  do
+      Class.new do
+        attr_reader :message
+
+        def initialize
+          @message = nil
+        end
+
+        def class
+          OpenStruct.new(name: 'ClassName')
+        end
+
+        def process(message)
+          @message = message
+        end
+      end.new
+    end
+    let(:message) { double(:message) }
+
+    before do
+      allow(Datadog.tracer).to receive(:trace).and_call_original
+    end
+
+    it 'uses Datadog tracer' do
+      handle
+
+      expect(Datadog.tracer).to have_received(:trace).with('ClassName',
+        hash_including(service: 'hutch', span_type: 'rabbitmq'))
+    end
+
+    it 'processes the message' do
+      expect {
+        handle
+      }.to change { klass.message }.from(nil).to(message)
+    end
+  end
+end