vpa-admission-controller: Add observability and alerting

[ialidzhikov]

How to categorize this issue?

/area auto-scaling
/kind enhancement

What would you like to be added:

• Configure prometheus to scrape the vpa-admission-controller metrics: Scrape vpa-admission-controller metrics with prometheus #10033
• Add plutono dashboards based on the metrics
• Add alerts

Why is this needed:
In a recent VPA related outage (ref kubernetes/autoscaler#6884), we noticed that we are missing observability for the vpa-admission-controller component. For example:

• we don't have a panel showing the response times of the component
• we don't have alerts if the vpa-admission-controller response time > 10s (the webhook's timeout).

[voelzmo]

/assign