Use __host__ label in promtail config

[james-callahan]

From https://grafana.com/docs/loki/latest/clients/promtail/scraping/#kubernetes-discovery

Note that while Promtail can utilize the Kubernetes API to discover pods as targets, it can only read log files from pods that are running on the same node as the one Promtail is running on. Promtail looks for a host label on each target and validates that it is set to the same hostname as Promtail's (using either $HOSTNAME or the hostname reported by the kernel if the environment variable is not set).
This means that any time Kubernetes service discovery is used, there must be a relabel_config that creates the intermediate label host from __meta_kubernetes_pod_node_name

We don't currently set the __host__ label. Though I think we still get host filtering via https://github.com/grafana/loki/blob/c53457feb95a871e7494d22c5e072004874a197c/clients/pkg/promtail/targets/file/filetargetmanager.go#L107-L117