feat: add http.target to otelmux (#1)

instrumentation/github.com/gorilla/mux/otelmux/internal/semconvutil/httpconv.go

@@ -330,7 +330,7 @@ func (c *httpConv) ServerRequest(server string, req *http.Request) []attribute.K
 	net.sock.host.port              The request doesn't have access to the underlying socket.
 
 	*/
-	n := 4 // Method, scheme, proto, and host name.
+	n := 5 // Method, scheme, proto, and host name.
 	var host string
 	var p int
 	if server == "" {
@@ -384,6 +384,14 @@ func (c *httpConv) ServerRequest(server string, req *http.Request) []attribute.K
 	attrs = append(attrs, c.scheme(req.TLS != nil))
 	attrs = append(attrs, c.NetConv.HostName(host))
 
+	if req.URL != nil {
+		attrs = append(attrs, c.HTTPTargetKey.String(req.URL.RequestURI()))
+	} else {
+		// This should never occur if the request was generated by the net/http
+		// package. Fail gracefully, if it does though.
+		attrs = append(attrs, c.HTTPTargetKey.String(req.RequestURI))
+	}
+
 	if hostPort > 0 {
 		attrs = append(attrs, c.NetConv.HostPort(hostPort))
 	}