ddtrace/tracer: Add normalization of span duration

ddtrace/tracer/span.go

@@ -369,9 +369,16 @@ func (s *span) finish(finishTime int64) {
 	s.context.finish()
 }
 
+func normalizeSpan(s *span) {
+	if s.Duration < 0 {
+		s.Duration = 0
+	}
+}
+
 // newAggregableSpan creates a new summary for the span s, within an application
 // version version.
 func newAggregableSpan(s *span, cfg *config) *aggregableSpan {
+	normalizeSpan(s)
 	var statusCode uint32
 	if sc, ok := s.Meta["http.status_code"]; ok && sc != "" {
 		if c, err := strconv.Atoi(sc); err == nil {

ddtrace/tracer/span_test.go

@@ -619,6 +619,20 @@ func TestSpanLog(t *testing.T) {
 	})
 }
 
+func TestNormalizeSpan(t *testing.T) {
+	t.Run("negativeduration", func(t *testing.T) {
+		actual := span{
+			Service:  "testservice",
+			Duration: -1,
+		}
+		normalizeSpan(&actual)
+		expected := span{
+			Service: "testservice",
+		}
+		assert.Equal(t, expected, actual)
+	})
+}
+
 func BenchmarkSetTagMetric(b *testing.B) {
 	span := newBasicSpan("bench.span")
 	keys := "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"