Testing Spot: Add check for missing index that causes high CPU usage

Hi, it me. I'm adding a check to try out the Spot helper script.

Thanks @dstull for the environment instructions!

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Adding a new check

Verification steps for review

Author checklist

• After opening the MR:
  • Set it to the current milestone
  • Ask the Maintainer from the Reviewer roulette suggestion for review

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Reviewer checklist

• I followed the verification steps and confirm the functionality of the new check
• This check does only perform read operations
• This check does not output more than necessary on stdout for the check to function
• The message explains what it means when this check does not pass
• The workaround_url provides actionable information/steps for affected users
• This check is not using the Rails console/runner, or has Maintainer approval for doing so
• If this is a breaking change check:
  • It has the corresponding xx_breaking_changes tag (xx being the major release version for the change)
  • The workaround_url goes to the entry on the https://docs.gitlab.com/update/deprecations/ page
  • The ref_url goes to the deprecation issue linked from that entry
  • The title is the same as that entry
  • The version_started is equal to the announcement_milestone of the deprecation
  • The version_fixed is equal to the removal_milestone of the deprecation