Add Gauge For CheckMySQL Running

[GuptaManan100]

Description

We have been seeing a failure of DemotePrimary wherein it just gets stuck indefinitely. The failure was traced to a call to acquiring a lock which was held by the CheckMySQL function which too is stuck indefinitely. This PR aims to add a gauge which tells us if CheckMySQL is running or not. If the gauge is constantly 1 for more than 10 minutes, then it is an indication that something is wrong and we can take a look.

Related Issue(s)

Checklist

• "Backport me!" label has been added if this change should be backported
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Deployment Notes

[vitess-bot]

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