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

Seems for some reason we must exclude NaN values from charts. We should document why.

values.filter(([, value]) => !Number.isNaN(value))

Original discussion

The following discussion from !16530 (merged) should be addressed:

• @pslaughter started a discussion: (+1 comment)

  quesiton (non-blocking): Can you add a comment why we have to filter out NaN's? 😅

  If I'm missing something obvious, feel free to just add a comment to this thread (not the code).