Fix /api/v3 data update_every after filtering

[ktsaou]

Summary

• move the global update_every assignment so only instances with queried metrics update it
• keep metadata for filtered instances untouched so dashboards still expose available filters

Why

• previously /api/v3/data reported the minimum cadence of every instance in scope, even ones filtered out of the actual query, because metadata collection runs before filters
• this made the top-level db.update_every disagree with tier 0 when dashboards sliced by instance or label

Testing

• not run