[vtadmin] vtexplain lock

[ajm188]

Description

(I took the easy way out, because the longer-term fix is much more, uh, involved 😅)

This PR adds a synchronization lock to concurrent vtexplain requests. I also took this opportunity to give us some rudimentary insight into how long we're stuck waiting on that lock, as well as to ensure we clean up the execution environment after each request.

Related Issue(s)

• This is the short-term fix described in [vtadmin|vtexplain] vtexplain is not safe for concurrent use #7723. I'll tackle the vtexplain refactor later, at which point we can remove this lock.

Checklist

• Should this PR be backported? no
• Tests were added or are not required no
• Documentation was added or is not required

Deployment Notes

Impacted Areas in Vitess

Components that this PR will affect:

• Query Serving
• VReplication
• Cluster Management
• Build/CI
• VTAdmin

[doeg]

Wow, this is a tricky bug that seems like it would have caused vexing problems down the road, so thanks for snagging it early. Your write-up in #7723 is great, too.