add an integration-ish test w/actual exporter

Also, lean into spec's let.

processor/baggage/test/opentelemetry/processor/baggage/baggage_span_processor_test.rb

@@ -25,51 +25,43 @@
 
 describe OpenTelemetry::Processor::Baggage::BaggageSpanProcessor do
   let(:processor) { OpenTelemetry::Processor::Baggage::BaggageSpanProcessor.new }
-  let(:exporter) { TEST_EXPORTER }
-
-  before do
-    exporter.reset
-  end
+  let(:span) { Minitest::Mock.new }
+  let(:context_with_baggage) { OpenTelemetry::Baggage.set_value('a_key', 'a_value') }
 
   describe '#on_start' do
-    before do
-      @span = Minitest::Mock.new
-      @context_with_baggage = OpenTelemetry::Baggage.set_value('a_key', 'a_value')
-    end
-
     it 'adds current baggage keys/values as attributes when a span starts' do
-      @span.expect(:add_attributes, @span, [{ 'a_key' => 'a_value' }])
+      span.expect(:add_attributes, span, [{ 'a_key' => 'a_value' }])
 
-      processor.on_start(@span, @context_with_baggage)
+      processor.on_start(span, context_with_baggage)
 
-      @span.verify
+      span.verify
     end
 
     it 'does not blow up when given nil context' do
-      processor.on_start(@span, nil)
+      processor.on_start(span, nil)
       assert true # nothing above raised an exception
     end
     it 'does not blow up when given nil span' do
-      processor.on_start(nil, @context_with_baggage)
+      processor.on_start(nil, context_with_baggage)
       assert true # nothing above raised an exception
     end
     it 'does not blow up when given nil span and context' do
       processor.on_start(nil, nil)
       assert true # nothing above raised an exception
     end
     it 'does not blow up when given a context that is not a Context' do
-      processor.on_start(@span, :not_a_context)
+      processor.on_start(span, :not_a_context)
       assert true # nothing above raised an exception
     end
     it 'does not blow up when given a span that is not a Span' do
-      processor.on_start(:not_a_span, @context_with_baggage)
+      processor.on_start(:not_a_span, context_with_baggage)
       assert true # nothing above raised an exception
     end
   end
 
   describe 'satisfies the SpanProcessor duck type with no-op methods' do
     it 'implements #on_finish' do
-      processor.on_finish(@span)
+      processor.on_finish(span)
       assert true # nothing above raised an exception
     end
 
@@ -81,4 +73,23 @@
       _(processor.shutdown).must_equal(OpenTelemetry::SDK::Trace::Export::SUCCESS)
     end
   end
+
+  describe 'integration test with an exporter' do
+    let(:tracer) { OpenTelemetry.tracer_provider.tracer('🧳') }
+    let(:exporter) { TEST_EXPORTER }
+
+    before do
+      exporter.reset
+    end
+
+    it 'adds baggage attributes to spans' do
+      tracer
+        .start_span('integration test span', with_parent: context_with_baggage)
+        .finish
+
+      _(exporter.finished_spans.size).must_equal(1)
+      _(exporter.finished_spans.first.name).must_equal('integration test span')
+      _(exporter.finished_spans.first.attributes).must_equal('a_key' => 'a_value')
+    end
+  end
 end