release-23.1: log: add buffer format options

[blathers-crl]

Backport 1/1 commits from #103463 on behalf of @xinhaoz.

/cc @cockroachdb/release

---

This commit adds an option to format the logging sink
buffers on flush.

New option (under buffering in the config file):

• format: string that is either 'json-array' or 'newline',
  defaulting to 'newline'.

Format options:

• json-array: adds '[' at the start of the buffer output and ']'
  at the end. Separates entries in the buffer with ",".
• newline (default): separates entries in the buffer with "\n".

This change was motivated by the desire to be able to send batched logs
to the Datadog API, which expects them to be in a JSON array when multiple
logs are present. Note that If buffering is off, it is still valid to
send a single JSON log to the Datadog logs API.

Part of: #103477

Release note (cli change): New log config option for the
buffering field to allow buffer output to formatted as
JSON arrays. This can be useful for APIs requring json array
format, such as the Datadog logs API.
New fields:

• format: 'json-array' or 'newline'.
  Descriptions:
• json-array: adds '[' at the start of the buffer output and ']'
  at the end. Separates entries in the buffer with ",".
• newline (default): separates entries in the buffer with "\n".

---

Release justification: high priority need for feature

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• 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.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is 

[dhartunian]

Reviewed 1 of 10 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @xinhaoz)