tracer: dual sampling, make sure priorities -1 and 2 are handled

tracer/span.go

@@ -315,13 +315,13 @@ func (s *Span) Tracer() *Tracer {
 	return s.tracer
 }
 
-// SetSamplingPriority sets the sampling priority.
-// Default is 0, any higher value is interpreted as a hint on
-// how interesting this span is, and should be kept by the backend.
+// SetSamplingPriority sets the sampling priority. Possible values are:
+// -1: The user asked not to keep the trace;
+// 0: The sampler automatically decided not to keep the trace;
+// 1: The sampler automatically decided to keep the trace;
+// 2: The user asked to keep the trace.
 func (s *Span) SetSamplingPriority(priority int) {
-	if priority >= 0 {
-		s.SetMetric(samplingPriorityKey, float64(priority))
-	}
+	s.SetMetric(samplingPriorityKey, float64(priority))
 }
 
 // HasSamplingPriority returns true if sampling priority is set.

tracer/span_test.go

@@ -265,37 +265,15 @@ func TestSpanSamplingPriority(t *testing.T) {
 	assert.Equal(span.HasSamplingPriority(), childSpan.HasSamplingPriority())
 	assert.Equal(span.GetSamplingPriority(), childSpan.GetSamplingPriority())
 
-	span.SetSamplingPriority(0)
-	assert.Equal(0.0, span.Metrics["_sampling_priority_v1"], "key has been deleted")
-	assert.Equal(0, span.GetSamplingPriority(), "by default, sampling priority is 0")
-	childSpan = tracer.NewChildSpan("my.child", span)
-	assert.Equal(span.Metrics["_sampling_priority_v1"], childSpan.Metrics["_sampling_priority_v1"])
-	assert.Equal(span.HasSamplingPriority(), childSpan.HasSamplingPriority())
-	assert.Equal(span.GetSamplingPriority(), childSpan.GetSamplingPriority())
-
-	span.SetSamplingPriority(-1)
-	assert.Equal(0.0, span.Metrics["_sampling_priority_v1"], "key has been deleted")
-	assert.Equal(0, span.GetSamplingPriority(), "by default, sampling priority can't be negative")
-	childSpan = tracer.NewChildSpan("my.child", span)
-	assert.Equal(span.Metrics["_sampling_priority_v1"], childSpan.Metrics["_sampling_priority_v1"])
-	assert.Equal(span.HasSamplingPriority(), childSpan.HasSamplingPriority())
-	assert.Equal(span.GetSamplingPriority(), childSpan.GetSamplingPriority())
-
-	span.SetSamplingPriority(1)
-	assert.Equal(1.0, span.Metrics["_sampling_priority_v1"], "sampling priority is now 1")
-	assert.Equal(1, span.GetSamplingPriority(), "sampling priority is now 1")
-	childSpan = tracer.NewChildSpan("my.child", span)
-	assert.Equal(span.Metrics["_sampling_priority_v1"], childSpan.Metrics["_sampling_priority_v1"])
-	assert.Equal(span.HasSamplingPriority(), childSpan.HasSamplingPriority())
-	assert.Equal(span.GetSamplingPriority(), childSpan.GetSamplingPriority())
-
-	span.SetSamplingPriority(42)
-	assert.Equal(42.0, span.Metrics["_sampling_priority_v1"], "sampling priority works for values above 1")
-	assert.Equal(42, span.GetSamplingPriority(), "sampling priority works for values above 1")
-	childSpan = tracer.NewChildSpan("my.child", span)
-	assert.Equal(span.Metrics["_sampling_priority_v1"], childSpan.Metrics["_sampling_priority_v1"])
-	assert.Equal(span.HasSamplingPriority(), childSpan.HasSamplingPriority())
-	assert.Equal(span.GetSamplingPriority(), childSpan.GetSamplingPriority())
+	for _, priority := range []int{-1, 0, 1, 2, 999} {
+		span.SetSamplingPriority(priority)
+		assert.True(span.HasSamplingPriority())
+		assert.Equal(priority, span.GetSamplingPriority())
+		childSpan = tracer.NewChildSpan("my.child", span)
+		assert.Equal(span.Metrics["_sampling_priority_v1"], childSpan.Metrics["_sampling_priority_v1"])
+		assert.Equal(span.HasSamplingPriority(), childSpan.HasSamplingPriority())
+		assert.Equal(span.GetSamplingPriority(), childSpan.GetSamplingPriority())
+	}
 }
 
 type boomError struct{}