ui: downsample SQL transaction metrics using MAX

[dhartunian]

Previously, we were using the default downsampling behavior of the
timeseries query engine for "Open SQL Transactions" and "Active SQL
Statements" on the metrics page in DB console. This led to confusion
when zooming in on transaction spikes since the spike would get larger
as the zoom got tighter.

This PR changes the aggregation function to use MAX to prevent this
confusion.

Resolves: #71827

Release note (ui change): Open SQL Transactions and Active SQL
Transactions are downsampled using MAX instead of AVG and will more
accurately reflect narrow spikes in transaction counts when looking and
downsampled data.

[cockroach-teamcity]

This change is 

[maryliag]

FYI, there was a PR merged yesterday with a change on the Active Statements, so you might want to rebase to make sure your changes work as expected with the change from https://github.com//pull/75815/files#diff-399fe90fa99139ea49c301594f8b7c699ab0304b48080b8b01ed8198a23a154cR66

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

[dhartunian]

TFTR! bors r=maryliag

[dhartunian]

bors r=maryliag

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from 86008a8 to blathers/backport-release-21.1-76348: POST https://api.github.com/repos/cockroachlabs/cockroach/merges: 403 Resource not accessible by integration []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 21.1.x failed. See errors above.

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error creating merge commit from 86008a8 to blathers/backport-release-21.2-76348: POST https://api.github.com/repos/cockroachlabs/cockroach/merges: 403 Resource not accessible by integration []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 21.2.x failed. See errors above.

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[craig]

Build succeeded:

• GitHub CI (Cockroach)