Elasticsearch: Monitor the max opened shards on a cluster

Monitor the number of shards opened in a cluster and raise an alert if it reaches a defined threshold compared to the max allowed :

Current cout:

curl -s http://192.168.100.61:9200/_stats\?pretty | jq '._shards'
{
  "total": 2934,
  "successful": 2934,
  "failed": 0
}

Max rules:

[1000|cluster.max_shards_per_nodes setting] * number of nodes

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Migrated from T3223 (view on Phabricator)