vpa-recommender: Group prometheus and external-metrics flags to dedicated var blocks

[ialidzhikov]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Right now, it is clear where the prometheus and external-metrics flags start but it is not clear where they end.
For example, --memory-saver is not a prometheus history provider related flag but it appears so.
This PR uses the already existing pattern of grouping flags into dedicated var blocks (see the rest of the flags; there are other groups like aggregation configuration flags and post processor flags).

Which issue(s) this PR fixes:

N/A

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

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

[raywainman]

/lgtm

Thanks for the cleanup

[adrianmoisey]

/lgtm

I'm a huge fan of changes like this, they seem super small, but make the project that much easier to work on. Over time these add up making for a nice neat easy to work on project. Thanks for doing it!

[voelzmo]

/approve

Thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ialidzhikov, voelzmo

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• vertical-pod-autoscaler/OWNERS [voelzmo]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment