DOCS-14819 Add SEARCH_META variable that populates from mongot

Author: kanchana-mongodb

source/reference/aggregation-variables.txt

@@ -107,6 +107,18 @@ MongoDB offers the following system variables:
 
      - One of the allowed results of a :pipeline:`$redact` expression.
 
+   * - .. variable:: SEARCH_META 
+
+     - A variable that stores the metadata results of an Atlas Search 
+       query. In a :pipeline:`$project` stage, a field set to the 
+       variable :variable:`$$SEARCH_META` returns the :atlas:`metadata 
+       results </reference/atlas-search/query-syntax/#metadata-result-types>`
+       for the query.
+
+       For an example of its usage, see Atlas Search :atlas:`facet 
+       </reference/atlas-search/facet/#search_meta-aggregation-variable>` 
+       and :atlas:`count </reference/atlas-search/counting/#search_meta-aggregation-variable>`.
+
 .. seealso::
 
    - :expression:`$let`