chore: add metrics test for NiFi 2.x

[xeniape]

Description

Adds NiFi 2.x test for metrics endpoint

Local

--- PASS: kuttl (396.09s)
    --- PASS: kuttl/harness (0.00s)
        --- PASS: kuttl/harness/smoke_v2_nifi-v2-2.4.0_use-zookeeper-manager-false_zookeeper-3.9.3_openshift-false_listener-class-cluster-internal (152.28s)
        --- PASS: kuttl/harness/smoke_v2_nifi-v2-2.4.0_use-zookeeper-manager-true_zookeeper-3.9.3_openshift-false_listener-class-cluster-internal (209.86s)
        --- PASS: kuttl/harness/smoke_v2_nifi-v2-2.4.0_use-zookeeper-manager-false_zookeeper-3.9.3_openshift-false_listener-class-external-unstable (171.56s)
        --- PASS: kuttl/harness/smoke_v2_nifi-v2-2.4.0_use-zookeeper-manager-true_zookeeper-3.9.3_openshift-false_listener-class-external-unstable (186.21s)

Openshift

--- PASS: kuttl (259.52s)
    --- PASS: kuttl/harness (0.00s)
        --- PASS: kuttl/harness/smoke_v2_nifi-v2-2.4.0_use-zookeeper-manager-true_zookeeper-3.9.3_openshift-true_listener-class-external-unstable (249.49s)
PASS

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

Author

• Integration tests passed (for non trivial changes)

Reviewer

• Code contains useful comments
• Code contains useful logging statements
• (Integration-)Test cases added
• Documentation added or updated. Follows the style guide.
• Changelog updated
• Cargo.toml only contains references to git tags (not specific commits or branches)

Acceptance

• Feature Tracker has been updated
• Proper release label has been added
• Links to generated (nightly) docs added
• Release note snippet added
• Add type/deprecation label & add to the deprecation schedule
• Add type/experimental label & add to the experimental features tracker

[sbernauer]

We want authenticated metrics for zecurity!
Oh you have a cert? I don't care about it's contents - here are the metrics xD