transmit event key from input topic to output topic in kafka

[eastmon94]

What this PR does / why we need it:
When doing stream serving with Kafka, we cannot receive event keys although it is required.
So we just need to send the event key from input topic to output topic.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[seldondev]

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[seldondev]

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[ukclivecox]

/test notebooks

[ukclivecox]

/test integration

[ukclivecox]

Thanks @eastmon94 can you explain in a bit more detail the addition. Are you saying your Kafka rejects new messages without a given request or you need to ensure it keeps the same Key?

[eastmon94]

Thanks @cliveseldon. There are cases where a key is needed when using the message received in the output topic as input. In my case, the result of the model should be post-processed with Beam and Flink, but in the case of the 'ReadFromKafka' operation, the key must be included in the message. So, I tried to simply pass the key entered in the input topic to the output topic.

[ukclivecox]

/test integration

[ukclivecox]

/test integration

[ukclivecox]

/test notebooks

[ukclivecox]

/test integration

[axsaucedo]

/test integration

[axsaucedo]

/test integration

[axsaucedo]

/test integration

[ukclivecox]

@eastmon94 Thanks!