Fix tracing duplicated headers

middlewares/tracing/carrier.go

@@ -0,0 +1,25 @@
+package tracing
+
+import "net/http"
+
+// HTTPHeadersCarrier custom implementation to fix duplicated headers
+// It has been fixed in https://github.com/opentracing/opentracing-go/pull/191
+type HTTPHeadersCarrier http.Header
+
+// Set conforms to the TextMapWriter interface.
+func (c HTTPHeadersCarrier) Set(key, val string) {
+	h := http.Header(c)
+	h.Set(key, val)
+}
+
+// ForeachKey conforms to the TextMapReader interface.
+func (c HTTPHeadersCarrier) ForeachKey(handler func(key, val string) error) error {
+	for k, vals := range c {
+		for _, v := range vals {
+			if err := handler(k, v); err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}