title: How to represent a JSON object variable with Proto definition
body: <p>I want to define a request message in gRPC which should have a Json Object as a field
  For e.g.</p>
  
  <pre><code>message UserRequest{
      string name = 1;
      string city = 2;
      string email = 3;
      metainfo = 4;//A Json Object variable which can have any number of elements
  }
  </code></pre>
  
  <p>How do I represent the metainfo property within proto definition?
  I have tried using below definition but it didn't work.</p>
  
  <pre><code>message UserRequest{
      string name = 1;
      string city = 2;
      string email = 3;
      google.protobuf.Any metainfo = 4;
  }
  </code></pre>