Analytics Dashboards: Improve metric table trend indicator tooltip for metrics with no data in time period

The following discussion from !215797 (merged) should be addressed:

• @ekigbo started a discussion: (+4 comments)

  nitpick: in this case is this because there were no duo sessions in the time period? I wonder if we can be a bit clearer in our wording, its not clear (to me) why there would be a division by zero at this point.

  

Problems with current tooltip text

• Too technical: "Division by zero" is a mathematical/programming concept that many users won't immediately understand in this context
• No context: It doesn't explain why there's a division by zero or what metric can't be calculated
• Unhelpful: Users can't take any action or gain insight from this message

Proposal

Update the UI text to:

"Value can't be calculated due to insufficient data. Change calculation requires at least six months of historical data."