feat: enable built in Prometheus servlet

[razvan]

Description

Fixes #694

CI https://testing.stackable.tech/view/02%20Operator%20Tests%20(custom)/job/hdfs-operator-it-custom/24/

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• Changes are OpenShift compatible
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• Code contains useful comments
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Acceptance

• Feature Tracker has been updated
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[razvan]

Release Notes

The built-in Prometheus servlet is now enabled by default and metrics are available under the /prom path of all UI services.
The metrics exposed by the JMX exporter are now considered deprecated and will be removed in a future release.

[dervoeti]

Just found a small typo, otherwise LGTM once the test passes (seems to be hanging because of external problems?).

Nit pick, feel free to ignore since it doesn't seem to cause problems:
The items in the expected_metrics list are used as regular expressions, it works, but it would be "more correct" if the curly braces were escaped.
Alternative: Maybe assert metric in response.text would also work, then re would not be needed at all. It's less strict though since it doesn't require the string to be at the beginning of the line.

[razvan]

Maybe assert metric in response.text would also work, then re would not be needed at all. It's less strict though since it doesn't require the string to be at the beginning of the line.

Yeah I saw that and forgot about it but now that you mentioned it ... I geeked out a little.

[razvan]

I ran the tests locally because it was faster:

--- PASS: kuttl (692.02s)
    --- PASS: kuttl/harness (0.00s)
        --- PASS: kuttl/harness/smoke_hadoop-3.4.1_zookeeper-3.9.3_zookeeper-latest-3.9.3_number-of-datanodes-1_datanode-pvcs-2hdd-1ssd_listener-class-cluster-internal_openshift-false (96.57s)
        --- PASS: kuttl/harness/smoke_hadoop-3.4.1_zookeeper-3.9.3_zookeeper-latest-3.9.3_number-of-datanodes-2_datanode-pvcs-default_listener-class-cluster-internal_openshift-false (127.70s)
        --- PASS: kuttl/harness/smoke_hadoop-3.4.1_zookeeper-3.9.3_zookeeper-latest-3.9.3_number-of-datanodes-2_datanode-pvcs-default_listener-class-external-unstable_openshift-false (128.61s)
        --- PASS: kuttl/harness/smoke_hadoop-3.4.1_zookeeper-3.9.3_zookeeper-latest-3.9.3_number-of-datanodes-2_datanode-pvcs-2hdd-1ssd_listener-class-external-unstable_openshift-false (146.90s)
        --- PASS: kuttl/harness/smoke_hadoop-3.4.1_zookeeper-3.9.3_zookeeper-latest-3.9.3_number-of-datanodes-2_datanode-pvcs-2hdd-1ssd_listener-class-cluster-internal_openshift-false (521.51s)
        --- PASS: kuttl/harness/smoke_hadoop-3.4.1_zookeeper-3.9.3_zookeeper-latest-3.9.3_number-of-datanodes-1_datanode-pvcs-default_listener-class-cluster-internal_openshift-false (97.19s)
        --- PASS: kuttl/harness/smoke_hadoop-3.4.1_zookeeper-3.9.3_zookeeper-latest-3.9.3_number-of-datanodes-1_datanode-pvcs-default_listener-class-external-unstable_openshift-false (99.51s)
        --- PASS: kuttl/harness/smoke_hadoop-3.4.1_zookeeper-3.9.3_zookeeper-latest-3.9.3_number-of-datanodes-1_datanode-pvcs-2hdd-1ssd_listener-class-external-unstable_openshift-false (94.74s)
PASS

[dervoeti]

Thanks, LGTM!