Elasticsearch service should provide a better way of registering custom ElasticsearchJS client "actions"

[azasypkin]

Once we merge #28344, core ElasticsearchService will be used to access Elasticsearch from both new and legacy platform. There is a bunch of places in Kibana where we create cluster clients with custom ElasticsearchJS client "actions". These actions are configured via plugins property of ElasticsearchClientConfig that has direct access to the underlying ElasticsearchJS client. E.g.:

const cluster = server.plugins.elasticsearch.createCluster('admin', {
  plugins: [monitoringBulk],
});

............monitoringBulk............

export function monitoringBulk(Client, _config, components) {
  Client.prototype.monitoring = components.clientAction.namespaceFactory();
  Client.prototype.monitoring.prototype.bulk = components.clientAction.factory({
    params: {
      system_id: {
        type: 'string'
      },
    },
    urls: [{
      fmt: '/_monitoring/<%=type%>/bulk',
      req: { type: { type: 'string' } }
    }],
    needBody: true,
    method: 'POST'
  });
}

This is basically a hole we left open because of BWC reasons and we should find a better way to accomplish the same thing with more controlled and convenient API that doesn't expose ElasticsearchJS client internals.

[elasticmachine]

Pinging @elastic/kibana-platform

[epixa]

The new elasticsearch-js client supports registering actions on an existing client. We should prioritize moving to that client interface so creating custom client classes and modifying prototypes is not necessary.

[azasypkin]

That's awesome! There is definitely some non-zero amount of work is necessary on our side to support that though, so I'll keep this open.

[mshustov]

should be done in #35508