[BUG] Kafka *ByteRate metrics are non-zero for idle pipelines

[cmanning09]

Describe the bug
While my pipeline is idle the kafka source plugin continuously reports byte rate data when there is no data flowing from the kafka cluster to my pipeline. This is seen for both incomingByteRate and outgoingByteRate.

Ideally I would expect this to be zero during idle periods.

[kkondaka]

incomingByteRate and outgoingByteRate are obtained from the Kafka Client Library by doing consumer.metrics(). I think they are non-zero because there are house-keeping messages like heart-beat messages are sent between the client and the server even when there is no regular traffic. DataPrepper just exposes whatever value is returned by consumer.metrics()