kvprober: write structured logs

[joshimhoff]

Is your feature request related to a problem? Please describe.
Today, the kvprober logs are text-based. These are hard to parse programatically. This makes it hard to analyze the logs programmatically, and also write alerts (we have time-series metrics for alerting but for single range issue a log-based alert may be the move).

Describe the solution you'd like
We could do structured logging via pkg/util/log/eventpb.

Describe alternatives you've considered
Only alternative I can think of is not doing anything.

Additional context
N/A

Jira issue: CRDB-12067

[github-actions]

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