[FEATURE] Support _plugins/_ism/explain API on per cluster metrics monitor

[spapadop]

Is your feature request related to a problem?
We want to be able to monitor the ISM policies situation (e.g., failures) with OpenSearch monitors. Keeping in mind the "per cluster metrics" type, supporting _plugins/_ism/explain API would be really beneficial on that regards.

What solution would you like?
Add GET _plugins/_ism/explain API on "per cluster metrics monitor" type.

What alternatives have you considered?
Have an external cron job doing GET _plugins/_ism/explain on all OpenSearch clusters, store the result in a centralised monitoring OpenSearch cluster and create regular monitors on top of those logs. It's too much hassle when "per cluster metrics" monitor type exists.

Do you have any additional context?

[dblock]

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