Datatype should not be send with every message.

Hi @SeppPenner,

As per specification, the datatype should only be send with the BIRTH messages. I've already changed the code so it looksup the knwon datatype from the known metrics dictionary in my own fork.

> [tck-id-payloads-metric-datatype-not-req] The datatype SHOULD NOT be included with metric definitions in NDATA, NCMD, DDATA, and DCMD messages.