fix: add units to the yAxis label and the legend in the performance dashboard

What does this MR do?

Adds units (GB, cores, etc) to the charts in the performance dashboard.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #59325 (closed)