title: Test coverage of protobuff messages
body: <p>I've got a .proto file with a few 
  messages which looks something like this, </p>
  
  <pre><code>Message messageA{
    required double value =1;
    //more fields
  } 
  
  Message messageB{
     required int32 value =1;
     //more fields
  } 
  </code></pre>
  
  <p>I'd like to verify that each message added to the file will have a "value" field. 
  If it doesn't, the proper result will be a failing unit test. </p>
  
  <p>Is there a better way to do this without 
  hoping the next programmer will add messages instances to a test? </p>
  
  <p>Thanks. </p>