[SLO] Improve custom equation validation

[simianhacker]

🍒 Summary

Currently the custom equation field on the Timeslice Metric and Custom Metric only validates the string using a REGEX to ensure the user doesn't use any invalid characters: A-Z, +, -, /, *, (, ), ?, !, &, :, |, >, <, =. This doesn't stop the user from creating an invalid equation, even though it validates the REGEX, like INVALID.

🎯 Acceptance Criteria

• When a user types valid characters but not a valid equation, we should mark the equation as "invalid" when ES is returning an error for that invalid equation.
• Update the GetPreviewData API to return an optional error when the query fails. { data: Data[], error?: string }

🧠 Additional Thoughts

We should use the preview request to validate the equation since it runs the actual equation via painless. If we don't get a valid response, we should set the field to "invalid". Ideally, users wouldn't save an equation which doesn't work in the preview, we just need to enforce that.

[elasticmachine]

Pinging @elastic/obs-ux-management-team (Team:obs-ux-management)

[maryam-saeidi]

There is also this ticket based on @mgiota's comment.

Can we implement this logic in a package or somewhere common and use it for SLO and the Custom threshold rule type? Or do these two cases have different criteria?

[mgiota]

@maryam-saeidi I think implementing this somewhere common is a good idea! I see you are working on Custom Threshold improvements, so we could consider if any of these improvements are applicable to the Custom metric SLI.

[mgiota]

I see @benakansara has similar issues with the custom equation validation, so we should definitely tackle this somewhere common.