Scans a Datadog query string for @-prefixed attribute names used in filter position (e.g. @type:foo or @type IN (...)).

Datadog metric filters strip the leading @ from log facets when log-based metrics are generated — the metric tag for log facet @type is just type. A query that filters on @type against a log-based metric is rejected by the SLO API with 400: numerator query is invalid.

Returns a deduplicated list of @-prefixed attribute names found.