text: Title: make gRPC DocumentArrayProto in Java
  Body: I want to use the gRPC protocol with a language other than Python.
  
  so I follow the instruction (https://docs.jina.ai/fundamentals/client/third-party-client/):
  
  
  "Download the two proto definition files: jina.proto and docarray.proto from [github](https://github.com/jina-ai/jina/tree/master/jina/proto) (be sure to use the latest release branch)
  Compile them with [protoc](https://grpc.io/docs/protoc-installation/) and precise to which programming language you want to compile them.
  Add the generated files to your project and import them in your code.
  You should finally be able to communicate with your Flow using the gRPC protocol. You can find more information on the gRPC message and service that you can use to communicate in the [Protobuf documentation](https://docs.jina.ai/proto/docs/).
  "
  
  
  I generate the Java class with proto files.
  But It didn't say how to build the proto DocumentArrayProto!
  Docarray.DocumentArrayProto da = Docarray.DocumentArrayProto.parseFrom("byte array here")
  "byte array here" for what? what's the format of the string?
  Or I should use the builder? And how?
  Please show me some example on how to build the DocumentArrayProto 
  with the given data like:
  text
  image
  tensor
  ...
  
  Thanks!
  
  
  
  