[kafka_consumer] Partition assignment strategy should be configurable

[dhoppe]

Feature Request

According to the following link, Kafka supports multiple partition assignment strategies:

• https://cwiki.apache.org/confluence/display/KAFKA/KIP-49+-+Fair+Partition+Assignment+Strategy

Proposal:

According to the following link, Sarama is able to choose between three different partition assignment strategies. It would be great, if this could be configured via Telegraf.

• https://github.com/Shopify/sarama/blob/d6c39e51b1aa805d23878ebe101a30675dc17866/balance_strategy.go

Current behavior:

Only one consumer is assigned to a partition of the Kafka topic.

Desired behavior:

Multiple consumers should be able to consume from the same partition of the Kafka topic.

Use case:

In some cases the data is not balanced very well netween the partitions of a Kafka tpic. It could take really long to reduce the lag of a single partition. It would be great, if multiple consumers could consume data from a single partition (roundrobin).