PR #110391 - ui: use MAX downsampler for sql connection rate

[cockroach-teamcity]

Related PR: cockroachdb/cockroach#110391
Commit: cockroachdb/cockroach@a508484
Epic: none

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Release note (ui change): The "SQL Connection Rate" metric on the SQL
Dashboard is downsampled using the MAX function instead of SUM. This
improves situations where zooming out would cause the connection rate
to increase for downsampled data.