ddtrace/tracer: add Measured StartSpanOption (#591)

Author: jdgumz

Diff for: ddtrace/tracer/option.go

@@ -315,6 +315,11 @@ func SpanType(name string) StartSpanOption {
 	return Tag(ext.SpanType, name)
 }
 
+// Measured marks this span to be measured for metrics and stats calculations.
+func Measured() StartSpanOption {
+	return Tag(keyMeasured, 1)
+}
+
 // WithSpanID sets the SpanID on the started span, instead of using a random number.
 // If there is no parent Span (eg from ChildOf), then the TraceID will also be set to the
 // value given here.

Diff for: ddtrace/tracer/span.go

@@ -344,4 +344,5 @@ const (
 	keyHostname                = "_dd.hostname"
 	keyRulesSamplerAppliedRate = "_dd.rule_psr"
 	keyRulesSamplerLimiterRate = "_dd.limit_psr"
+	keyMeasured                = "_dd.measured"
 )

Diff for: ddtrace/tracer/tracer_test.go

@@ -187,6 +187,9 @@ func TestTracerStartSpan(t *testing.T) {
 			ext.PriorityAutoReject,
 			ext.PriorityAutoKeep,
 		}, span.Metrics[keySamplingPriority])
+		// A span is not measured unless made so specifically
+		_, ok := span.Meta[keyMeasured]
+		assert.False(ok)
 	})
 
 	t.Run("priority", func(t *testing.T) {
@@ -201,6 +204,12 @@ func TestTracerStartSpan(t *testing.T) {
 		assert.Equal(t, "db.query", span.Name)
 		assert.Equal(t, "/home/user", span.Resource)
 	})
+
+	t.Run("measured", func(t *testing.T) {
+		tracer := newTracer()
+		span := tracer.StartSpan("/home/user", Measured()).(*span)
+		assert.Equal(t, 1.0, span.Metrics[keyMeasured])
+	})
 }
 
 func TestTracerRuntimeMetrics(t *testing.T) {
@@ -243,6 +252,7 @@ func TestTracerStartSpanOptions(t *testing.T) {
 		ResourceName("test.resource"),
 		StartTime(now),
 		WithSpanID(420),
+		Measured(),
 	}
 	span := tracer.StartSpan("web.request", opts...).(*span)
 	assert := assert.New(t)
@@ -252,6 +262,7 @@ func TestTracerStartSpanOptions(t *testing.T) {
 	assert.Equal(now.UnixNano(), span.Start)
 	assert.Equal(uint64(420), span.SpanID)
 	assert.Equal(uint64(420), span.TraceID)
+	assert.Equal(1.0, span.Metrics[keyMeasured])
 }
 
 func TestTracerStartChildSpan(t *testing.T) {