DOCS-14819 Add SEARCH_META variable that populates from mongot

Signed-off-by: Jeff Allen <[email protected]>

Closes #5888

DOCS-15819 updates for tech review feedback

Signed-off-by: Jeff Allen <[email protected]>

Co-authored-by: Kanchana Sekhar <[email protected]>

Author: jeff-allen-mongo

source/reference/aggregation-variables.txt

@@ -107,6 +107,19 @@ 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:`Atlas 
+       Search </atlas-search/>` query. In all supported aggregation 
+       pipeline stages, 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`