Enable backwards-compatibility with controller-runtime v0.15

[annasong20]

What this PR does / why we need it:

This PR makes the code backwards-compatible with versions of controller-runtime before 0.16, which introduced the metrics/server package that kdp now uses. The PR uses //go:build to segregate handling of the metrics configuration depending on the version of controller-runtime.

Special notes for your reviewer:

N/A

Additional documentation:

The following links may be useful references

• When kdp updated to controller-runtime v0.16: https://github.com/kubernetes-sigs/kubebuilder-declarative-pattern/pull/352
• Previous example of //go:build usage: https://github.com/kubernetes-sigs/kubebuilder-declarative-pattern/pull/342
• Usage of metrics/server in codebase: https://github.com/search?q=repo%3Akubernetes-sigs%2Fkubebuilder-declarative-pattern+%22sigs.k8s.io%2Fcontroller-runtime%2Fpkg%2Fmetrics%2Fserver%22&type=code

[k8s-ci-robot]

Skipping CI for Draft Pull Request. If you want CI signal for your change, please convert it to an actual PR. You can still manually trigger a test run with /test all

[annasong20]

/cc @yuwenma @haiyanmeng

[yuwenma]

/lgtm

[justinsb]

Thanks @annasong20 - this is nice!

/approve /lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: annasong20, justinsb

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/kubernetes-sigs/kubebuilder-declarative-pattern/blob/master/OWNERS)~~ [justinsb] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment