fix: default span kind (#1034)

* fix: default span kind to follow spec

* fix: set default span kind in internal create api

* fix: add simple test for span kind default

* chore: appease the cop

Author: robertlaurin

sdk/lib/opentelemetry/sdk/trace/tracer_provider.rb

@@ -126,8 +126,9 @@ def add_span_processor(span_processor)
         end
 
         # @api private
-        def internal_create_span(name, kind, trace_id, parent_span_id, attributes, links, start_timestamp, parent_context, instrumentation_library) # rubocop:disable Metrics/MethodLength
+        def internal_create_span(name, kind, trace_id, parent_span_id, attributes, links, start_timestamp, parent_context, instrumentation_library) # rubocop:disable Metrics/MethodLength, Metrics/AbcSize
           trace_id ||= @id_generator.generate_trace_id
+          kind ||= :internal
           result = @sampler.should_sample?(trace_id: trace_id, parent_context: parent_context, links: links, name: name, kind: kind, attributes: attributes)
           span_id = @id_generator.generate_span_id
           if result.recording? && !@stopped

sdk/test/opentelemetry/sdk/trace/tracer_test.rb

@@ -24,6 +24,10 @@
       _(tracer.start_root_span(nil).name).wont_be_nil
     end
 
+    it 'defaults to span kind internal when not specified' do
+      _(tracer.start_root_span(nil).kind).must_equal(:internal)
+    end
+
     it 'returns a valid span' do
       span = tracer.start_root_span('root')
       _(span.context).must_be :valid?