Legacy client counts are low since May '23

[edugfilho]

All channels are impacted. GLAM client counts dropped significantly around May '23.
Some facts:

• That's around the date we had an incident involvingclients_histogram_aggregates
• The client counts shown on GLAM (that are low) are calculated per probe, with a statistical trick
• GLAM also calculates the amount of users per channel/build/os. Maybe we should use that instead?

[edugfilho]

A bug in the sql script that builds clients_histogram_aggregates_v2 allows the table to be updated without any data. This occurs when a day is skipped in the execution and is likely to be the reason why the counts are low. I'm backfilling nightly data for that table. Should get done tomorrow. Then I'll proceed to fix beta and release