Document why the `NaN` values are excluded from monitor charts.

What does this MR do?

Add a couple of comments to explain why NaN is filtered out.

Screenshots

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

Closes #34955 (closed)