Only take metrics snapshot for pods that has VPA enabled.

[figo]

What type of PR is this?

/kind bug

What this PR does / why we need it:

When memorySaveMode enabled, VPA today still consume several Gb memory when running in a large cluster with 1k+ pods, the reason is that it is taking metrics snapshot for all pods regardless of whether VPA enabled for it.

This PR is to only take metrics snapshot for pods that has VPA enabled when memorySaveMode is on.

In our testing with AWS as backend, it is reducing the memory footprint of VPA pod from 4Gb+ to under 2 Gb.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

None

VPA will only consume small amount of memory when memorySaveMode is on.

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

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[gjtempleton]

@voelzmo Any chance you could have a look at this?

[voelzmo]

Sorry, I dropped the ball on this a bit. I will be able to continue working on this next week. Sorry for the delay.