feat: add custom Prometheus metrics for CPU allocation monitoring

[AutuSnow]

Add 7 custom Prometheus metrics to the /metrics endpoint:

• dra_cpu_allocated_cpus (gauge)
• dra_cpu_available_cpus (gauge)
• dra_cpu_reserved_cpus (gauge)
• dra_cpu_resource_claims_active (gauge)
• dra_cpu_prepare_claims_success_total (counter)
• dra_cpu_prepare_claims_error_total (counter)
• dra_cpu_unprepare_claims_total (counter)
• dra_cpu_prepare_claim_duration_seconds (histogram)

#73

[k8s-ci-robot]

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[AutuSnow]

cc @ffromani

[ffromani]

Add 7 custom Prometheus metrics to the /metrics endpoint:

* dra_cpu_allocated_cpus (gauge)

* dra_cpu_available_cpus (gauge)

* dra_cpu_reserved_cpus (gauge)

* dra_cpu_resource_claims_active (gauge)

* dra_cpu_prepare_claims_success_total (counter)

* dra_cpu_prepare_claims_error_total (counter)

* dra_cpu_unprepare_claims_total (counter)

* dra_cpu_prepare_claim_duration_seconds (histogram)

please document the metrics in the README, adding another section.
It would be best to have a way to introspect the reported metrics programmatically (e.fg. new command line flag).
Finally, I think is gonna be worthy to have a histogram to expose how many cpus pods are using (e.g. 1-2, 3-4, 5-9 ...)

Last but not least, metrics are arguably a component API. It's so early in the life of the project that adding metrics is more worthy than get them right (if we can at all with the little data we have now), so please make sure to document that these metrics are provisional, not GA, and that we reserve the option to change them in the future.

[pohly]

please make sure to document that these metrics are provisional, not GA, and that we reserve the option to change them in the future.

All metrics start out as alpha, are documented as such, and graduate once we have confidence that we got them right, independent of the feature which introduced them.

[AutuSnow]

cc @ffromani @pravk03

[k8s-ci-robot]

PR needs rebase.

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[AutuSnow]

@pravk03 @ffromani Hi, could you please take a look at this PR? I've added some monitoring metrics that I feel are quite important (this hasn't been discussed), so I'd like to seek your opinions. If there are any missing or unnecessary metrics, I'm more than happy to modify my current PR

[k8s-ci-robot]

@AutuSnow: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-dra-driver-cpu-e2e-device-mode-individual-amd64	c02bd6c	link	true	/test pull-dra-driver-cpu-e2e-device-mode-individual-amd64
pull-dra-driver-cpu-e2e-device-mode-grouped-amd64	c02bd6c	link	true	/test pull-dra-driver-cpu-e2e-device-mode-grouped-amd64
pull-dra-driver-cpu-e2e-device-mode-individual-arm64	c02bd6c	link	true	/test pull-dra-driver-cpu-e2e-device-mode-individual-arm64
pull-dra-driver-cpu-e2e-device-mode-grouped-arm64	c02bd6c	link	true	/test pull-dra-driver-cpu-e2e-device-mode-grouped-arm64

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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