sql: add plan gist to sampled query telemetry log

[cockroach-teamcity]

Exalate commented:

Related PR: cockroachdb/cockroach#83027
Commit: cockroachdb/cockroach@39a562c

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Release note (sql change): The sampled query telemetry log now includes
a plan gist field. The plan gist field provides a compact representation
of a logical plan for the sampled query, the field is written as a
base64 encoded string.

Jira Issue: DOC-4663

[exalate-issue-sync]

Andrew Feierabend (andf-crl) commented:
No docs for telemetry changes.