[ART-380] Fix tracer leaving background writer threads

This was causing a race in
`bundle exec rspec --seed 50224 spec/ddtrace/tracer_integration_spec.rb`
exposing the issue in ART-380 where multiple writer background threads
would trigger initialization of metrics.

This does not fix the underlying issue causing
`ArgumentError: wrong number of arguments (given 2, expected 0)`
(see next commit for that).

spec/ddtrace/tracer_integration_spec.rb

@@ -9,6 +9,10 @@
   subject(:tracer) { described_class.new(writer: FauxWriter.new) }
   let(:spans) { tracer.writer.spans(:keep) }
 
+  after do
+    tracer.shutdown! # Ensure no state gets left behind
+  end
+
   def sampling_priority_metric(span)
     span.get_metric(Datadog::Ext::DistributedTracing::SAMPLING_PRIORITY_KEY)
   end