Resolves #14303.

As reported in #14303, when the Elasticsearch monitoring reporter in libbeat sends a bulk API request to Elasticsearch, and that request fails, the errors are currently swallowed. This is because the actual response code for the bulk API request is 200 OK; the actual errors are embedded in the request's response body.

This PR teaches the Elasticsearch monitoring reporter to parse the bulk API response and log any errors. For the parsing, the same code as the Elasticsearch output is reused.

Testing this PR

1. Start up Elasticsearch with security enabled. Make sure you know the password for the elastic superuser.

2. Create a role that grants necessary privileges for managing and writing to metricbeat-* indices.

   curl -s -u elastic -H 'Content-Type: application/json' 'http://localhost:9200/_security/role/mb_writer' -d '{ "cluster": [ "monitor", "manage_ilm", "manage_index_templates" ], "indices": [ { "names": [ "metricbeat-*" ], "privileges": [ "all" ] } ] }'
   

3. Create a user with the above role.

   curl -s -u elastic -H 'Content-Type: application/json' 'http://localhost:9200/_security/user/mb_writer' -d '{ "password": "mb_writer", "roles": [ "mb_writer" ] }'
   

4. Build Metricbeat with this PR.

   cd $GOPATH/src/github.com/elastic/beats/metricbeat
   mage build
   

5. Start Metricbeat with monitoring enabled and specifying the credentials of the above user for the elasticsearch output.

   ./metricbeat -e -E output.elasticsearch.username=mb_writer -E output.elasticsearch.password=mb_writer -E monitoring.enabled=true
   

6. Verify that metricbeat-* indices are being created and populated in Elasticsearch but no .monitoring-beats-* indices are being created.

   curl -s -u elastic 'http://localhost:9200/_cat/indices'
   

7. Verify that there are warnings in the log like so:

   2019-11-01T08:57:43.910-0700    WARN    elasticsearch/client.go:258     monitoring bulk item insert failed (i=0, status=403): {"type":"security_exception","reason":"action [indices:admin/create] is unauthorized for user [mb_writer]"}