[vtadmin] experimental tabletdebug

[ajm188]

Description

This PR adds an experimental, http-only endpoint to vtadmin for making passthrough requests to a tablet's /debug/vars endpoint.

Example:

amason@some-host:~/vitess$ curl -s "${VTADMIN_URL}/api/experimental/tablet/${TABLET_HOSTNAME}/debug/vars | jq '.result.BuildGitBranch' 
HEAD

Related Issue(s)

Checklist

• Tests were added or are not required – not required
• Documentation was added or is not required

Deployment Notes

[doeg]

I love it 😈

We talked about this previously, but for posterity: is there anything I should keep in mind in querying this endpoint? Beyond the usual considerations like... y'know, "do not query this in an infinite loop".

A reasonable front-end implementation would poll this endpoint around once every second (per client) -- I believe we arrived at "this should not be a problem, and if it is a problem, we can tune it later", which works for me.

[ajm188]

🤔 I don't think so!! It's still experimental, so let's just keep going with "this should not be a problem, and if it is a problem, we can tune it later"