[release-17.0] vtexplain: Ensure memory topo is set up for throttler (#15279)

[vitess-bot]

Description

This is a backport of #15279

[vitess-bot]

Hello @dbussink, there are conflicts in this backport.

Please address them in order to merge this Pull Request. You can execute the snippet below to reset your branch and resolve the conflict manually.

Make sure you replace origin by the name of the vitessio/vitess remote

git fetch --all
gh pr checkout 15283 -R vitessio/vitess
git reset --hard origin/release-17.0
git cherry-pick -m 1 c4afae2950d57d5ddd5e31281ab7c69167d701c6

[vitess-bot]

Review Checklist

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