Simple HTTP status code healthcheck endpoint

[archoversight]

Describe the feature:

While I am familiar with /api/status it returns a 200 check even while service is degraded and metric beat or other connections will fail.

Would it be possible to get an HTTP endpoint that will not return a 200 OK until Kibana is actually ready to serve traffic. Having load balancers parse the output from /api/status is not a feature available to most of them to make sure that Kibana is not degraded.

Describe a specific use case for the feature:

• Healtcheck in a docker-compose setup so that metric beat won't get started until Kibana is ready for it (metric beat currently exits with status code 1 if it fails to connect to Kibana)
• Healthcheck for ALB/load balancers so that Kibana instances don't get traffic until they are actually ready to serve said traffic (so users don't get 503 service unavailable or search errors)

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[lukeelmers]

Closing as a duplicate of #46984. Thanks for writing up these use cases -- I'll copy them over to that issue.