Update guide for monitoring with Elastic Agent to include also `` role

[andsel]

Tell us about the issue

Description:
Following the guide Collect Logstash monitoring data for stack monitoring it's requested to create a user with role remote_monitoring_collector , but that's not enough. With just that role, the ElasticAgent logs the following erro message:

{
"log.level":"error",
"@timestamp":"2024-05-03T08:44:22.747Z",
"message":"failed to perform any bulk index operations: 403 Forbidden: {\"error\":{\"root_cause\":[{\"type\":\"security_exception\",\"reason\":\"action [indices:data/write/bulk] is unauthorized for user [logstash_collector] with effective roles [remote_monitoring_collector] (assigned roles [create,create_doc,delete,index,write] were not found), this action is granted by the index privileges [create_doc,create,delete,index,write,all]\"}],\"type\":\"security_exception\",\"reason\":\"action [indices:data/write/bulk] is unauthorized for user [logstash_collector] with effective roles [remote_monitoring_collector] (assigned roles [create,create_doc,delete,index,write] were not found), this action is granted by the index privileges [create_doc,create,delete,index,write,all]\"},\"status\":403}",
"component":
   {"binary":"filebeat",
     "dataset":"elastic_agent.filebeat",
     "id":"log-default",
      "type":"log"},
"log":
    {"source":"log-default"},
"log.logger":"elasticsearch",
"log.origin":
   {"file.line":258,
    "file.name":"elasticsearch/client.go"},
"service.name":"filebeat",
"ecs.version":"1.6.0",
"ecs.version":"1.6.0"
}

but roles named create,create_doc,delete,index,write cited in (assigned roles [create,create_doc,delete,index,write] were not found), doesn't exists, are just privileges.

Adding also the role remote_monitor_agent fixes the problem

URL:
https://www.elastic.co/guide/en/logstash/current/monitoring-with-elastic-agent.html

Anything else?