Include fetched offset metadata retrieved with admin.fetchOffsets

[JaapRood]

Kafka supports a nullable string of metadata to be stored with a committed offset. KafkaJS actually supports this with it's protocol definitions, but admin.fetchOffsets filters it out just before returning the retrieved offsets to us.

[JaapRood]

As you might see from the tests: it only asserts metadata of null is forwarded. While consumerGroup will pass on any metadata and write it, that method isn't exposed anywhere at the moment. admin.setOffsets actually spawns a Consumer and relies on the auto commit mechanism to commit a resolved offset after seeking to it. See #378.

[Nevon]

Can you make sure to update the return type for fetchOffsets as well? https://github.com/tulios/kafkajs/blob/master/index.d.ts#L272

EDIT: Being handled in #390