The metrics datasource defaulted to http://127.0.0.1:<port>, which only
reached the store when Grafana and VictoriaMetrics ran on the same host.
Swarm-tier services may not assume colocation (#4499), so the default is
now https://<swarm.victoriametrics.domain>, the gateway vhost the store's
host already serves. That vhost's query routes carry no auth_request, and
the Grafana container already trusts the hive CA for its authelia token
exchange, so no credential is added.
The logs datasource stays on loopback: the log store's query route needs
a bearer token Grafana does not hold yet.
Part of #3742 (item 4, metrics).
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