It's a completely different problem because by default Prometheus does not shard anything so you're bound to a single instance, where ES and Kafka are cluster based.

Out of interest what do you find hard about running ElasticSearch clusters?

In my experience ES has been one of the easiest clustered / highly available and sharded systems I've ever run - especially for how incredibly performant and reliable it is.

I've generally found that beyond right sizing your nodes, indexes and shard configuration - it pretty much just works without ever really having issues.