fix(experiments): Standardize trends cont. exposure query logic

[danielbachhuber]

See #26713

Changes

Standardizes trends continuous exposure query logic to use $feature_flag_called or the custom exposure event, not a count of the count event.

Also ensures filterTestAccounts is applied to default trend exposure queries.

How did you test this code?

Tests should pass. This branch hasn't actually been hit since #27432, though.

[andehen]

Looks good! 🧹

Tested it locally with two different trend metrics.

Yeah, after the mentioned PR the removed branch should no longer have been visited (at least by users in prod). I haven't see this bug since then either.

[danielbachhuber]

Yeah, after the mentioned PR the removed branch should no longer have been visited (at least by users in prod).

Actually, fwiw, I think the branch was still visited. Your PR modifies prepared_count_query but the exposure query uses the original self.query.count_query.