[Metricbeat] standardize checks of r.Event inside loops

[fearful-symmetry]

This is a more specific continuation of #11666
It looks like the best way to handle calling r.Event inside a loop is to check the bool return value, and just return from Fetch if we get a false. This will only happen when the metricset is closed. This obviously isn't a huge issue, but it would be nice to standardize how we handle this.

• all of postgres
• docker/image
• all of vsphere
• ceph/monitor_health
• jolokia/jmx
• mongodb/collstats
• most of system
• consul/agent
• kafka/partition [metricbeat] migrate kafka to reporterV2 with error return #11868
• kvm/dommemstat
• aerospike/namespace
• prometheus/collector
• windows/perfmon
• windows/service
• couchbase/bucket
• couchbase/node
• aws (Check return from report.Event and migrate to use ReportingMetricSetV2Error #11775)