Here is a working snippet:
ManagedChannel channel =
ManagedChannelBuilder.forAddress("<your-name>.grpc.us-east-1.aws.thegaas.com", 443).useTransportSecurity().build();
DgraphStub stub = DgraphGrpc.newStub(channel);
Metadata metadata = new Metadata();
metadata.put(
Metadata.Key.of("authorization", Metadata.ASCII_STRING_MARSHALLER), "<api-key-here>");
stub = MetadataUtils.attachHeaders(stub, metadata);
return new DgraphClient(stub);