KAFKA-17347: Add missing client-metrics option to kafka-configs.sh

[AndrewJSchofield]

When KIP-714 was developed, the entity type of client-metrics was added to the kafka-configs.sh tool. The idea was to have two forms of specifying the name and type of a client metrics config resource, either --entity-type client-metrics --entity-name NAME or --client-metrics NAME. This style of alias is used for all of the entity types. Unfortunately, the --client-metrics form was not implemented. This PR corrects that and adds more tests.

Committer Checklist (excluded from commit message)

• [ ] Verify design and implementation
• [ ] Verify test coverage and CI build status
• [ ] Verify documentation (including upgrade notes)