Prevent unnecessary sticking to the DB primary when recording experiments

What does this MR do?

Experiments can be run on high-traffic read-only pages and when a transaction is initiated, our load balancing code will direct all queries to the primary.

This update is a no-op most of the time so we skip the update when we detect that the attributes haven't changed.

ActiveRecord actually already does this under the hood but a #transaction call is still made in the process causing the undesired behavior with our load balancer.

More details in #324649 (closed)

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