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

#### 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:
```json
{
"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?**