Add default Telemetry SDK resource create method

The specification requires a Telemetry::SDK resource, this adds a
method for create it directly from the resource class.  This also
initializes it by default on the configurator.  The resource
setter on the configurator class will merge the default resource
with the one provided.

sdk/lib/opentelemetry/sdk/configurator.rb

@@ -16,7 +16,7 @@ class Configurator # rubocop:disable Metrics/ClassLength
       private_constant :USE_MODE_UNSPECIFIED, :USE_MODE_ONE, :USE_MODE_ALL
 
       attr_writer :logger, :http_extractors, :http_injectors, :text_extractors,
-                  :text_injectors, :resource
+                  :text_injectors
 
       def initialize
         @adapter_names = []
@@ -27,13 +27,17 @@ def initialize
         @text_injectors = nil
         @span_processors = []
         @use_mode = USE_MODE_UNSPECIFIED
-        @resource = OpenTelemetry::SDK::Resources::Resource.create
+        @resource = Resources::Resource.telemetry_sdk
       end
 
       def logger
         @logger ||= Logger.new(STDOUT)
       end
 
+      def resource=(new_resource)
+        @resource = @resource.merge(new_resource)
+      end
+
       # Install an instrumentation adapter with specificied optional +config+.
       # Use can be called multiple times to install multiple instrumentation
       # adapters. Only +use+ or +use_all+, but not both when installing

sdk/lib/opentelemetry/sdk/resources/resource.rb

@@ -11,7 +11,7 @@ module Resources
       # for which telemetry (metrics or traces) is reported.
       class Resource
         class << self
-          private :new # rubocop:disable Style/AccessModifierDeclarations
+          private :new
 
           # Returns a newly created {Resource} with the specified labels
           #
@@ -29,6 +29,14 @@ def create(labels = {})
 
             new(frozen_labels)
           end
+
+          def telemetry_sdk
+            create(
+              Constants::TELEMETRY_SDK_RESOURCE[:name] => 'opentelemetry',
+              Constants::TELEMETRY_SDK_RESOURCE[:language] => 'ruby',
+              Constants::TELEMETRY_SDK_RESOURCE[:version] => "semver:#{OpenTelemetry::SDK::VERSION}"
+            )
+          end
         end
 
         # @api private

sdk/test/opentelemetry/sdk/configurator_test.rb

@@ -15,6 +15,24 @@
     end
   end
 
+  describe '#resource=' do
+    let(:configurator_resource) { configurator.instance_variable_get(:@resource) }
+    let(:configurator_resource_labels) { configurator_resource.label_enumerator.to_h }
+    let(:expected_resource_labels) do
+      {
+        'telemetry.sdk.name' => 'opentelemetry',
+        'telemetry.sdk.language' => 'ruby',
+        'telemetry.sdk.version' => "semver:#{OpenTelemetry::SDK::VERSION}",
+        'test_key' => 'test_value'
+      }
+    end
+
+    it 'merges the resource' do
+      configurator.resource = OpenTelemetry::SDK::Resources::Resource.create('test_key' => 'test_value')
+      _(configurator_resource_labels).must_equal(expected_resource_labels)
+    end
+  end
+
   describe '#use' do
     it 'can be called multiple times' do
       configurator.use('TestAdapter', enabled: true)

sdk/test/opentelemetry/sdk/resources/resource_test.rb

@@ -41,6 +41,15 @@
     end
   end
 
+  describe '.telemetry_sdk' do
+    it 'returns a resource for the telemetry sdk' do
+      resource_labels = Resource.telemetry_sdk.label_enumerator.to_h
+      _(resource_labels['telemetry.sdk.name']).must_equal('opentelemetry')
+      _(resource_labels['telemetry.sdk.language']).must_equal('ruby')
+      _(resource_labels['telemetry.sdk.version']).must_match(/semver:\b\d{1,3}\.\d{1,3}\.\d{1,3}/)
+    end
+  end
+
   describe '#merge' do
     it 'merges two resources into a third' do
       res1 = Resource.create('k1' => 'v1', 'k2' => 'v2')