Added ServiceMonitor

[Vovcharaa]

Added ability to create ServiceMonitor for metrics scrapping.

Also, I believe it would be nice to deprecate prometheus key in values.yaml and move everything under metrics key.
Let me know whether to implement this change in this PR.

Tasks:

• [x] I've made changes
• [x] I've bumped chart's version in Chart.yaml
• [x] I've added changes to charts CHANGELOG.md
• [x] I've run helm-docs

[MaikuMori]

Also, I believe it would be nice to deprecate prometheus key in values.yaml and move everything under metrics key.

I see your point. I'd prefer to keep it as is because it mirrors Gotenberg's “modules” in the documentation. This way it's easier for me to flip between the docs and values, and I would hope that it's the same for users. Moreover, I prefer not to have breaking changes if we can avoid it.

It seems like it's failing tests. I can check what the issue is in a bit, but currently I'm a bit swamped.

I'm also wondering if it would make sense to add tests with Prometheus operator installed and metrics set to enable. Not necessary full e2e test, but just so that we can see that everything deploys without obvious errors.

Thank you, I definitely want to get this merged, after we deal with the issues.

[MaikuMori]

I've created a 1.7.0 milestone, I will get all issues/PRs into that release. Probably by creating a 1.7.0 pull request and merging everything there, and then merging that into master once done.