Custom Security Extension and Realm Work for Kibana Login

That doesn't work for login, which is in practice what you need for Kibana (API calls are far less infrequent than Elasticsearch). The token authentication provider performs login using user/pass credentials.
I do not believe this to be possible right now (*) - see also Authentication provider in Kibana

(*) At least, not easily. One can do it with legacy plugins if xpack security is disabled; It may also be possibly by a mix of legacy & new platform code, but I don't believe it can be done in a new-platform plugin. Please do let me know if I'm wrong about that.

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