Alert when detecting a specific number of shards on index

I would like to detect when a number of shards per index passes some threshold.

I use Metricbeat with Elasticsearch module to retrieve metrics data from indices and ElastAlert to send alerts when some number of shards per index is detected.

At first, I thought I could somehow use the _stats API or _settings endpoint in order to access the number of shards per index and run ElastAlert on this number. However, as far as I know, ElastAlert scans only the _doc section of indices, and not _stats or _settings .

As written in the docs of Elasticsearch module of Metricbeat, there is a elasticsearch.cluster.stats.indices.shards.count field which lists the number of shards for the whole cluster :

elasticsearch.cluster.stats.indices.shards.count
  type: long

  Total number of shards in cluster.

I wonder if there is something similar to that but for each index ? If not, what are the possible workarounds?