release-23.2: cli: create new tracing span in mt-start-sql

[blathers-crl]

Backport 1/1 commits from #113553 on behalf of @yuzefovich.

/cc @cockroachdb/release

---

This commit fixes a possible misuse of the tracing infrastructure with mt start-sql command. In particular, previously it was possible for a tracing span created (and finished) in runStartInternal to be used by a couple of things in makeTenantSQLServerArgs (main one being rpc.Context) that hold on to the context after the corresponding span has been finished. This is disallowed (since we reuse spans after Finish has been called on them) and was recently discovered. This problem is now fixed by deriving a child span only in mt start-sql code path that is never finished (since those users can be active throughout the whole lifetime of the tenant server).

The bug is pretty minor (I think only serverless is affected), so there is no release note.

Fixes: #113170.

Release note: None

---

Release justification: low-risk bug fix.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
• Backports should not change on-disk formats or node communication protocols.
• Backports should not add new functionality (except as defined
  here).
• Backports must not add, edit, or otherwise modify cluster versions; or add version gates.
• All backports must be reviewed by the owning areas TL and one additional
  TL. For more information as to how that review should be conducted, please consult the backport
  policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.
• Your backport must be accompanied by a post to the appropriate Slack
  channel (#db-backports-point-releases or #db-backports-XX-X-release) for awareness and discussion.

Also, please add a brief release justification to the body of your PR to justify this
backport.

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[jeffswenson]

LGTM

The fix is low risk and only impacts Serverless deployments.