With this change in place, the grafana service will signal
readiness to serve by writing "READY=1" to the path specified
through the NOTIFY_SOCKET environment variable. If this
environment variable is not present or empty, no notification
will happen. This notification is the standard systemd
mechanism for indicating a service is ready to serve. For
Grafana this may be a couple of seconds from startup due to
database migrations. This change also adjusts the Grafana
systemd service definition to make use of this feature.