Fixes#2015
## Changes
Based on discussions in the messaging workgroup and in issue #2015, this
PR proposes to remove `messaging.consumer.id`, and to replace both
`messaging.kafka.client_id` and `messaging.rocketmq.client_id` with a
generic `messaging.client_id`.
`messaging.consumer.id` is defined to always be set to the `client_id`
of the used messaging system, except for Kafka, where it was defined to
be a combination of `messaging.kafka.client_id` and
`messaging.kafka.consumer.group`, or just the latter if
`messaging.kafka.client_id` is not available. With this definition, the
semantics of `consumer.id` are different between messaging systems, and
even different for different Kafka scenarios.
The proposed `messaging.client_id` has consistent semantics ("an unique
client id, when it is available"), and can be used instead of
`messaging.consumer.id` in almost all cases.
In addition to have consistent semantics, this also simplifies the
semantic conventions, as instead of
`messaging.consumer.id`
`messaging.kafka.client_id`
`messaging.rocketmq.client_id`
there is now just:
`messaging.client_id`.
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