Add support for cancelling metrics search query

What does this MR do and why?

Make metrics search query cancellable. This is currently not hooked up to anything, but will be once the cancel button is added ( gitlab-org/opstrace/opstrace#2610 (closed)).

Closes gitlab-org/opstrace/opstrace#2672 (closed)

MR acceptance checklist

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Screenshots or screen recordings

No visual changes

How to set up and validate locally

Run rspec