Fix inconsistent total count on TopN events panel (#111256)

We can't display `response.totalCount` because it is the total number of
events the query returns. It doesn't take into account the aggregation.
It does include events with missing `stackedByField` and events that are
not included in the 10 top.

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>

Author: machadoum

x-pack/plugins/security_solution/public/common/containers/matrix_histogram/index.ts

@@ -141,7 +141,7 @@ export const useMatrixHistogram = ({
                   data: response.matrixHistogramData,
                   inspect: getInspectResponse(response, prevResponse.inspect),
                   refetch: refetch.current,
-                  totalCount: response.totalCount,
+                  totalCount: histogramBuckets.reduce((acc, bucket) => bucket.doc_count + acc, 0),
                   buckets: histogramBuckets,
                 }));
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