statshost: add loki data source to grafana configuration

If the loki role is also enabled on the same host as statshost-master,
then add a Loki data source to the Grafana configuration.

PL-132981

nixos/roles/statshost/default.nix

@@ -18,6 +18,8 @@ let
  cfgProxyLocation = config.flyingcircus.roles.statshost-location-proxy;
  cfgProxyRG = config.flyingcircus.roles.statshost-relay;
 
+ cfgLokiRG = config.flyingcircus.roles.loki;
+
  promFlags = [
  "--storage.tsdb.retention.time ${toString cfgStats.prometheusRetention}d"
  ];
@@ -517,12 +519,30 @@ in
  isDefault: true
  '';
  };
+
+ # support loki running on the same host as grafana in
+ # single-RG mode.
+ lokiDatasource = pkgs.writeTextFile {
+ name = "loki.yaml";
+ text = ''
+ apiVersion: 1
+ datasources:
+ - name: Loki
+ type: loki
+ access: proxy
+ orgId: 1
+ url: http://${config.networking.hostName}:3100
+ editable: false
+ isDefault: false
+ '';
+ };
  in ''
  rm -rf ${grafanaProvisioningPath}
  mkdir -p ${grafanaProvisioningPath}/dashboards ${grafanaProvisioningPath}/datasources
  ln -fs ${fcioDashboards} ${grafanaProvisioningPath}/dashboards/fcio.yaml
  ln -fs ${prometheusDatasource} ${grafanaProvisioningPath}/datasources/prometheus.yaml
-
+ '' + optionalString (cfgStatsRG.enable && cfgLokiRG.enable) ''
+ ln -fs ${lokiDatasource} ${grafanaProvisioningPath}/datasources/loki.yaml
  '';
 
  # Provide FC dashboards, and update them automatically.