Returns the available filter operators for each column in the specified audience. Use this endpoint before building a filter-based row query to discover which operators — such as EQUALS, CONTAINS, NOT_EQUALS are supported for each column. The response includes per-operator configuration: whether multiple values are supported (isMultipleSupported), the maximum number of allowed values (valueCountLimit), and the minimum character requirement per value for string-based operators (minCharLimit). Use this metadata to validate filter inputs client-side before sending a List Rows request. Returns 200 with a list of FilterMetadata objects keyed by columnId. Returns 404 if no audience matches the provided audienceId.