code: public Schema toSchema(Descriptor msgDescriptor) {
      Schema schema = new Schema();
  
      try {
        // Walk through all the possible fields in the message.
        for (FieldDescriptor fieldDescriptor : msgDescriptor.getFields()) {
          if (fieldDescriptor.getType() == FieldDescriptor.Type.MESSAGE) {
            schema.add(messageToFieldSchema(fieldDescriptor));
          } else {
            schema.add(singleFieldToFieldSchema(fieldDescriptor));
          }
        }
      } catch (FrontendException e) {
        LOG.warn("Could not convert descriptor " + msgDescriptor + " to schema", e);
      }
  
      return schema;
    }
docs: Turn a generic message descriptor into a Schema.  Individual fields that are enums
  are converted into their string equivalents.
  @param msgDescriptor the descriptor for the given message type.
  @return a pig schema representing the message.