I use a HTTP poller inverted. The outage text is a little bit confusing, isn't it? (see attachment)

<service name="ES-Cluster-Status-Yellow" interval="600000" user-defined="true" status="on">
<parameter key="invert-status" value="true"/>
<parameter key="retry" value="1"/>
<parameter key="timeout" value="3000"/>
<parameter key="port" value="9200"/>
<parameter key="resolve-ip" value="false"/>
<parameter key="nodelabel-host-name" value="true"/>
<parameter key="url" value="/_cluster/stats"/>
<parameter key="response" value="200-202,299"/>
<parameter key="response-text" value="~.status.:.(yellow)."/>
</service>