net-mgmt/grok_exporter: Update to 1.0.0.RC4

Update net-mgmt/grok_exporter to 1.0.0.RC4, set @sample on its config file,
and detach from the terminal when daemonizing.

PR: 244613
Event: September 2020 Bugathon
Approved by: dbaio (ports)
Approved by: Gasol Wu <gasol.wu@gmail.com> (maintainer, earlier version)
Sponsored by: Axcient