Change node memory query to use `max`

What does this MR do?

This came out of reviewing some odd data submissions from Usage Ping in #229457 (closed).

This MR represents the client-side changes for omnibus-gitlab!4420 (merged)

It just reflects a change in a Prometheus query name for Usage Ping that now records the max instead of the avg.

Some minor refactoring as well in that the helper method to construct the aggregation function name can now be either a max or avg.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

The only meaningful way to test this is in Omnibus:

• Test in Omnibus container