Report FQDN for `detected_hostname`

# Description
ECS changed the definition of host.name and encourages to use the fully qualified domain name for the host name.

All Elastic ECS producers should populate the host.name field with the lowercased FQDN from here forward.

This change is behind a feature flag on the Elastic agent (to avoid breaking changes for users).

~APM agents do not know about the above-mentioned setting / feature flag. Thus, this decision needs to happen on the APM server.
However, **APM agents would need to collect the host.domain information in addition to enable the APM server to construct the FQDN.**~

**IMPORTANT UPDATE**: We keep it simple for APM agents. This change only requires the APM agents to always report the FQDN (instead of the simple) for the `detected_hostname` field.

## Spec Issue
- [ ] https://github.com/elastic/apm/issues/794

## Agent Issues
- [ ] https://github.com/elastic/apm-agent-java/issues/3131
- [ ] https://github.com/elastic/apm-agent-dotnet/issues/2065
- [x] https://github.com/elastic/apm-agent-nodejs/issues/3310
- [ ] https://github.com/elastic/apm-agent-python/issues/1824
- [ ] https://github.com/elastic/apm-agent-go/issues/1425
- [ ] https://github.com/elastic/apm-agent-php/issues/943