Cherry-pick of PR #8445 to 6.x branch. Original message:

Porting over elastic/elasticsearch#33860 to the Metricbeat Elasticsearch module (X-Pack Monitoring code path).

This PR teaches Elasticsearch X-Pack Monitoring to collect cluster metadata, if any is set, and index it into cluster_stats docs in .monitoring-es-6-mb-*.

After this PR, cluster_stats docs in .monitoring-es-6-mb-* will contain an additional top-level cluster_settings field like so:

{
   ...
   "cluster_settings": {
     "cluster": {
       "metadata": {
         ...
       }
     }
   }
}