feat(inputs.kafka_consumer): Add message key as metric tags

### Use Case

Kafka headers and timestamp can be metric tag
Why not Kafka key?

https://github.com/influxdata/telegraf/pull/13924
https://github.com/influxdata/telegraf/pull/15790

### Expected behavior

I would like to copy Kafka message key as metric tag

### Actual behavior

By default copy Kafka message key as metric tag is not supported

### Additional info

_No response_

Start at the inputs.kafka_consumer entry point and inspect how Kafka headers and timestamps are copied into metric tags. Compare the linked pull requests for existing tag behavior, then determine the expected handling for the Kafka message key and verify that the key appears as a metric tag when configured.

Written by the indexing model from the issue text.