[Infrastructure UI] Refactor the use of reduces for the Metric Threshold Rule in 7.17

[simianhacker]

As a follow up to #121904 we need to refactor the execution code to remove the reduces in 7.17 since the original PR only targets the 8.0 branch. For this ticket, we are ONLY refactoring the three places referred in the original PR to eliminate the event loop blocking. We shouldn't add the new configuration settings or anything beyond the scope of removing the reduces.

[elasticmachine]

Pinging @elastic/infra-monitoring-ui (Team:Infra Monitoring UI)

[weltenwort]

Are you sure it's the reduce that's causing it? Or is it the fact that we're creating a new array in each iteration? Would mutating the accumulator yield the same speed-ups?

[simianhacker]

@weltenwort You're right, it's the spread... I just tested it between the for/of and reduce but modified the accumulator instead of using the spread and the performance gains are the same. I will update this PR to keep the reduces but eliminate the spread.

[simianhacker]

Although... I kind of prefer the for/of code because it's more concises.

[weltenwort]

Thanks for investigating further ❤️ reduce vs for is mostly a matter of taste and depends on whether a sequence of statement or a single expression is more appropriate. 🤷 Good to know we can choose either as long as we pay attention to the array creation.

[simianhacker]

Closed with #126545