Fix #17730 - Dev mode has new line (#18367)

* adding new line only in case of pretty in url not in dev mode

* change log added

.changelog/18367.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+dev-mode: Fix dev mode has new line in responses. Now new line is added only when url has pretty query parameter.
+```

agent/http.go

@@ -616,7 +616,9 @@ func (s *HTTPHandlers) marshalJSON(req *http.Request, obj interface{}) ([]byte,
 		if err != nil {
 			return nil, err
 		}
-		buf = append(buf, "\n"...)
+		if ok {
+			buf = append(buf, "\n"...)
+		}
 		return buf, nil
 	}