Adding docs on how to authenticate with prometheus to nifi 2.x.x metrics

[Maleware]

Description

Part of #704

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
- [ ] Changes are OpenShift compatible
- [ ] CRD changes approved
- [ ] CRD documentation for all fields, following the [style guide](https://docs.stackable.tech/home/nightly/contributor/docs/style-guide).
- [ ] Helm chart can be installed and deployed operator works
- [ ] Integration tests passed (for non trivial changes)
- [ ] Changes need to be "offline" compatible

# Reviewer
- [ ] Code contains useful comments
- [ ] Code contains useful logging statements
- [ ] (Integration-)Test cases added
- [ ] Documentation added or updated. Follows the [style guide](https://docs.stackable.tech/home/nightly/contributor/docs/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
- [ ] [Roadmap](https://github.com/orgs/stackabletech/projects/25/views/1) has been updated

[soenkeliebau]

Is this ready for review @Maleware ? I don't think you requested one yet..

[Maleware]

Yes and no.

It's the only working solution, but it will break stuff. I'd talk about it today to see if we want to make the efforts to fix it or leave as is. Depending on this, we can merge it or document a better way.

[Maleware]

needs to wait for https://github.com/stackabletech/decisions/issues/54 as this changes the regex expression

[Maleware]

after talking to @adwk67 we are not waiting on the decision of the service name. We will merge with the current implementation ( It's already with listener ) and rework if needed.

Moving to ready for review.

[razvan]

lgtm

[lfrancke]

Can you please link to the generated docs?

[Maleware]

Generated Docs:
https://docs.stackable.tech/home/nightly/nifi/usage_guide/monitoring/#_authentication_with_nifi_2_x_x