contrib/gin-gonic/gin: Extract parent span info from HTTP Headers

Use propagation features to try to get some span context in the incoming
request headers.

If the headers are not set, the comportment is the same as before

If a span is already present in the request's context, it will be
used primarily as the parent span.

Fixes #280

contrib/gin-gonic/gin/gintrace.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"strconv"
 
+	"gopkg.in/DataDog/dd-trace-go.v1/ddtrace"
 	"gopkg.in/DataDog/dd-trace-go.v1/ddtrace/ext"
 	"gopkg.in/DataDog/dd-trace-go.v1/ddtrace/tracer"
 
@@ -16,13 +17,17 @@ import (
 func Middleware(service string) gin.HandlerFunc {
 	return func(c *gin.Context) {
 		resource := c.HandlerName()
-		span, ctx := tracer.StartSpanFromContext(c.Request.Context(), "http.request",
+		opts := []ddtrace.StartSpanOption{
 			tracer.ServiceName(service),
 			tracer.ResourceName(resource),
 			tracer.SpanType(ext.AppTypeWeb),
 			tracer.Tag(ext.HTTPMethod, c.Request.Method),
 			tracer.Tag(ext.HTTPURL, c.Request.URL.Path),
-		)
+		}
+		if spanctx, err := tracer.Extract(tracer.HTTPHeadersCarrier(c.Request.Header)); err == nil {
+			opts = append(opts, tracer.ChildOf(spanctx))
+		}
+		span, ctx := tracer.StartSpanFromContext(c.Request.Context(), "http.request", opts...)
 		defer span.Finish()
 
 		// pass the span through the request context

contrib/gin-gonic/gin/gintrace_test.go

@@ -167,3 +167,27 @@ func TestGetSpanNotInstrumented(t *testing.T) {
 	response := w.Result()
 	assert.Equal(response.StatusCode, 200)
 }
+
+func TestExtractSpan(t *testing.T) {
+	assert := assert.New(t)
+	mt := mocktracer.Start()
+	defer mt.Stop()
+
+	r := httptest.NewRequest("GET", "/user/123", nil)
+	w := httptest.NewRecorder()
+
+	pspan := tracer.StartSpan("test")
+	tracer.Inject(pspan.Context(), tracer.HTTPHeadersCarrier(r.Header))
+
+	router := gin.New()
+	router.Use(Middleware("foobar"))
+	router.GET("/user/:id", func(c *gin.Context) {
+		span, ok := tracer.SpanFromContext(c.Request.Context())
+		assert.True(ok)
+
+		//Ensure current span if a child of the span injected in the HTTP request
+		assert.Equal(span.(mocktracer.Span).ParentID(), pspan.(mocktracer.Span).SpanID())
+	})
+
+	router.ServeHTTP(w, r)
+}