[Metrics UI] Threshold alert redundant groupBy throws a false positive on OR matches

[Zacqary]

This shouldn't happen. The user should only be seeing this warning for something like host.name: host-1.

We can fix this by changing the redundant groupBy check: https://github.com/Zacqary/kibana/blob/master/x-pack/plugins/infra/public/alerting/metric_threshold/components/expression.tsx#L263-L283

to use:

pattern: new RegExp(`{"match(_phrase)?":{"${group}":"(.*?)"}}`, "g");
// Add the "g" flag for a global search in order to count matches

and using alertParams.filterQuery.match(pattern).length === 1 instead of pattern.test(alertParams.filterQuery) as the boolean.

[elasticmachine]

Pinging @elastic/logs-metrics-ui (Team:logs-metrics-ui)

[Zacqary]

As per discussion on PR #116822, AC for this issue is NOT to update the logic to avoid these false positives, but to continue to display the warning and link to new documentation about groupBy.

The user may still need to be warned about using OR logic in their filterQuery, as they may not realize that this will limit their rule to only two groups.