Add custom prometheus metrics documentation

[geezyx]

Hello there!

I am a user of kubebuilder and recently was trying to figure out how to publish additional metrics from my controllers. I struggled to find documentation on how to achieve this, but did eventually figure it out from another PR (kubernetes-sigs/controller-runtime#132)

This PR adds a simple example of how one might add collectors and register them with the global registry.

I also updated reference.md which seemed to have some stale content.

I'd be happy to adjust the example if someone has a better approach or more generic way of presenting the idea to users for documentation purposes.

I intentionally didn't add a lot of extra explanation to the doc, since the example is pretty straightforward. Let me know if you want more context or examples.

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[DirectXMan12]

thanks!

/lgtm
/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: DirectXMan12, geezyx

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