title: Protobuf encodes int64 data type to string in json mapping
body: <p>I have a data source of proto serialized messages and I have the .proto file, so I know each field's type to-be.
  I deserialize a proto message like this:</p>
  <pre><code>from google.protobuf.json_format import MessageToJson
  data = MessageToJson(proto_obj, including_default_value_fields=True)
  </code></pre>
  <p>The problem is that every int64 field is converted to string type, and it's because of <a href="https://developers.google.com/protocol-buffers/docs/proto3#json" rel="nofollow noreferrer">protobuf json mapping</a></p>
  <p>I need to jsonize the data, and it's important to me to make the fields with default value present in the data, so I set the flag <code>including_default_value_fields=True</code> to consider this.</p>
  <blockquote>
  <p>If a field has the default value in the protocol buffer, it will be omitted in the JSON-encoded data by default to save space.</p>
  </blockquote>
  <p>The main struggle here is about preventing casting int64 types to string. Does anybody have any solution?
  Thank you in advance!</p>
  <p>PS: I'm using proto3.</p>