Add a metric for machine swap capacity

[iholder101]

While for memory there are metrics for both current consumption (container_memory_usage_bytes) and total machine capacity (machine_memory_bytes), for swap memory the only supported metric is for current consumption (container_memory_swap).

This PR adds the missing piece by adding machine_swap_bytes metric that would be helpful to monitor the total amount of swap memory available on the machine.

Fixes kubernetes/kubernetes#119424

[k8s-ci-robot]

Hi @iholder101. Thanks for your PR.

I'm waiting for a google member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[iwankgb]

/ok-to-test