Updating handling of sample priority extraction

This change updates the sample priority extraction logic to match what
is provided in Datadog's SDKs.  I.e. any positive inter value is treated
as a true value, any 0 or negative values is treated as a false value.
Non-integer values cause context extraction to fail with an error.

propagator.go

@@ -16,17 +16,20 @@ const (
 	priorityHeaderKey = "x-datadog-sampling-priority"
 	// TODO: other headers: https://github.com/DataDog/dd-trace-go/blob/4f0b6ac22e14082ee1443d502a35a99cd9459ee0/ddtrace/tracer/textmap.go#L73-L96
 
+	// These are typical sampling values for Datadog, but Datadog libraries actually support any integer values
+	// values >=1 mean the trace is sampled, values <= 0 mean the trace is not sampled
 	notSampled = "0"
 	isSampled  = "1"
 )
 
 var (
 	empty = trace.SpanContext{}
 
-	errMalformedTraceID     = errors.New("cannot parse Datadog trace ID as 64bit unsigned int from header")
-	errMalformedSpanID      = errors.New("cannot parse Datadog span ID as 64bit unsigned int from header")
-	errInvalidTraceIDHeader = errors.New("invalid Datadog trace ID header found")
-	errInvalidSpanIDHeader  = errors.New("invalid Datadog span ID header found")
+	errMalformedTraceID              = errors.New("cannot parse Datadog trace ID as 64bit unsigned int from header")
+	errMalformedSpanID               = errors.New("cannot parse Datadog span ID as 64bit unsigned int from header")
+	errInvalidTraceIDHeader          = errors.New("invalid Datadog trace ID header found")
+	errInvalidSpanIDHeader           = errors.New("invalid Datadog span ID header found")
+	errInvalidSamplingPriorityHeader = errors.New("invalid Datadog sampling priority header found")
 )
 
 // Propagator serializes Span Context to/from Datadog headers.
@@ -115,7 +118,11 @@ func extract(traceID, spanID, sampled string) (trace.SpanContext, error) {
 		}
 	}
 
-	if sampled == isSampled {
+	sampledInt, err := strconv.Atoi(sampled)
+	if err != nil {
+		return empty, errInvalidSamplingPriorityHeader
+	}
+	if sampledInt >= 1 {
 		scc.TraceFlags = trace.FlagsSampled
 	}
 

propagator_test.go

@@ -53,11 +53,8 @@ func TestExtractMultiple(t *testing.T) {
 		},
 		{
 			ddTraceIDSmall, ddParentIDSmall, "",
-			trace.SpanContextConfig{
-				TraceID: traceIDSmall,
-				SpanID:  spanIDSmall,
-			},
-			nil,
+			trace.SpanContextConfig{},
+			errInvalidSamplingPriorityHeader,
 		},
 		{
 			"", ddParentID, "",
@@ -79,6 +76,28 @@ func TestExtractMultiple(t *testing.T) {
 			trace.SpanContextConfig{},
 			errInvalidSpanIDHeader,
 		},
+		{
+			ddTraceID, ddParentID, "foo",
+			trace.SpanContextConfig{},
+			errInvalidSamplingPriorityHeader,
+		},
+		{
+			ddTraceID, ddParentID, "2",
+			trace.SpanContextConfig{
+				TraceID:    traceID,
+				SpanID:     spanID,
+				TraceFlags: trace.FlagsSampled,
+			},
+			nil,
+		},
+		{
+			ddTraceID, ddParentID, "-1",
+			trace.SpanContextConfig{
+				TraceID:    traceID,
+				SpanID:     spanID,
+			},
+			nil,
+		},
 	}
 
 	for _, test := range tests {