docs: Add disable serving metrics info if port 0

[mikesmithgh]

I was troubleshooting a couple things and wanted to disable prometheus metrics and came across the ability to disable it by setting the port to 0. See: https://github.com/kubernetes-sigs/controller-runtime/blob/master/pkg/manager/manager.go#L199

@grzesuav @AmitKumarDas I am thinking it might be better to default the value to 0 and have the user enable it if necessary. Thoughts?

[codecov]

Codecov Report

Merging #416 (a328188) into master (d5014ac) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #416   +/-   ##
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  Coverage   49.80%   49.80%           
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  Files          54       54           
  Lines        4883     4883           
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  Hits         2432     2432           
  Misses       2202     2202           
  Partials      249      249           

Flag	Coverage Δ
integration	42.96% <ø> (-0.11%)	⬇️
unit	28.73% <ø> (ø)

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[AmitKumarDas]

@mjsmith1028 I have some queries w.r.t changing the default behaviour of metrics. What issues do you face with current default metrics settings. It will be nice to stick to existing defaults (to avoid changes to all end user deployments) if it is not a bug.

[mikesmithgh]

@mjsmith1028 I have some queries w.r.t changing the default behaviour of metrics. What issues do you face with current default metrics settings. It will be nice to stick to existing defaults (to avoid changes to all end user deployments) if it is not a bug.

Hi @AmitKumarDas, I don't have any issues. I was just thinking to disable it by default because some users may not have prometheus setup. But, I agree that makes sense to leave it as is to avoid a breaking change if users are on the current default port 9999 👍 thanks!

[grzesuav]

🎉 This PR is included in version 2.1.0 🎉

The release is available on GitHub release

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