ducktape/metrics_reporter: Add a test for `has_kafka_gssapi `

[BenPope]

Test that the metrics reporter is reporting the correct metrics for has_kafka_gssapi when it is both enabled and disabled.

if we factor out the fetching of this metadata, then perhaps we could call that and have a stronger assertion in the kerberos tests where we can see the flag change rather than just exist?

Originally posted by @dotnwat in #8281 (comment)

JIRA Link: CORE-1143