Update example Prometheus queries

What does this MR do?

• Update metric names to match current release.
• Improve descriptions.
• Improve examples to show more query language functionality.

Memory:

• Show use of or function.
• Automatically select "MemAvailable" kernel metric if possible.

CPU:

• Show use of avg without ....
• Show CPU utilization per node.

Network:

• Show use of != label matching.
• Don't show "localhost" metrics.

What are the relevant issue numbers?

Fixes: https://gitlab.com/gitlab-org/gitlab-ce/issues/49849

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides