re-add spans using splat operator (#1580)

Author: plantfansam

sdk/lib/opentelemetry/sdk/trace/export/batch_span_processor.rb

@@ -125,7 +125,7 @@ def force_flush(timeout: nil) # rubocop:disable Metrics/MethodLength
                 dropped_spans = snapshot.shift(n)
                 report_dropped_spans(dropped_spans, reason: 'buffer-full', function: __method__.to_s)
               end
-              spans.unshift(snapshot) unless snapshot.empty?
+              spans.unshift(*snapshot) unless snapshot.empty?
               @condition.signal if spans.size > max_queue_size / 2
             end
           end

sdk/test/opentelemetry/sdk/trace/export/batch_span_processor_test.rb

@@ -201,14 +201,15 @@ def to_span_data
       test_exporter = TestExporter.new
       bsp = BatchSpanProcessor.new(test_exporter)
       bsp.on_finish(TestSpan.new)
+      bsp.on_finish(TestSpan.new)
       result = bsp.force_flush(timeout: 0)
 
       _(result).must_equal(TIMEOUT)
 
       _(test_exporter.failed_batches.size).must_equal(0)
       _(test_exporter.batches.size).must_equal(0)
 
-      _(bsp.instance_variable_get(:@spans).size).must_equal(1)
+      _(bsp.instance_variable_get(:@spans).size).must_equal(2)
     end
   end