Aggregate Documents

This endpoint performs aggregation operations on documents in a collection.

**Aggregation Framework**: Provides MongoDB-style aggregation operations:
- GROUP BY: Group documents by one or more fields
- Aggregations: COUNT, SUM, AVG, MIN, MAX, COUNT_DISTINCT, etc.
- Date Operations: Truncate or extract date parts for time-series analysis
- Filtering: Pre-aggregation filters (WHERE) and post-aggregation filters (HAVING)
- Sorting & Limiting: Control result ordering and size

**Counting documents that match a filter**: omit group_by and send only count
aggregations, for example
`{"aggregations": [{"function": "count", "alias": "total"}], "filters": {...}}`.
The response holds one row whose metrics carry the exact number of matching
documents (an empty `group`). This is the same count documents/list returns in
`pagination.total` when you send `"include_total": true`. Any other
aggregation needs at least one group_by field.

**Use Cases**:
- Count documents by feature type or collection
- Calculate daily/monthly processing statistics
- Analyze feature distributions and confidence scores
- Generate reports with multiple metrics

**Group values**: a group_by value comes back as a JSON number or boolean when the
namespace declares a payload index of type integer, float or bool on the field. Any
other field, including an undeclared numeric one, groups as strings.

**Note**: This endpoint works with both MongoDB and the vector store using the same interface.
The system automatically selects the appropriate aggregation provider based on
the underlying metadata store.