Add mem_total_bytes gauge to Prometheus

What does this MR do and why?

Adds Prometheus gauge for our already existing metric: mem_total_bytes. Having this in Kibana is useful, but we are limited only to the last 7 days. It would be useful to track trends and allocations per worker over 1GB for a longer period of time.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.