adding k to knn search (#2781) (#2782)

(cherry picked from commit 84399d0)

Co-authored-by: Laura Trotta <[email protected]>

Author: github-actions[bot]

output/openapi/elasticsearch-openapi.json

@@ -60,6 +60,8 @@ export interface KnnQuery extends QueryBase {
   query_vector_builder?: QueryVectorBuilder
   /** The number of nearest neighbor candidates to consider per shard */
   num_candidates?: integer
+  /** The final number of nearest neighbors to return as top hits */
+  k?: integer
   /** Filters for the kNN search query */
   filter?: QueryContainer | QueryContainer[]
   /** The minimum similarity for a vector to be considered a match */