Enable prometheus, add loki config example, we don't have a global log system so it's not useful though

nixos/server/package-configs/dashboard/loki/default.nix

@@ -0,0 +1,10 @@
+{
+  config,
+  pkgs,
+  ...
+}: {
+  services.loki = {
+    enable = true;
+    configFile = ./loki.yaml;
+  };
+}

nixos/server/package-configs/dashboard/loki/loki.yaml

@@ -0,0 +1,40 @@
+# Enables authentication through the X-Scope-OrgID header, which must be present
+# if true. If false, the OrgID will always be set to "fake".
+auth_enabled: false
+
+server:
+  http_listen_address: "0.0.0.0"
+  http_listen_port: 3100
+
+ingester:
+  lifecycler:
+    address: "127.0.0.1"
+    ring:
+      kvstore:
+        store: inmemory
+      replication_factor: 1
+    final_sleep: 0s
+  chunk_idle_period: 5m
+  chunk_retain_period: 30s
+
+schema_config:
+  configs:
+  - from: 2020-05-15
+    store: boltdb
+    object_store: filesystem
+    schema: v11
+    index:
+      prefix: index_
+      period: 168h
+
+storage_config:
+  boltdb:
+    directory: /tmp/loki/index
+
+  filesystem:
+    directory: /tmp/loki/chunks
+
+limits_config:
+  enforce_metric_name: false
+  reject_old_samples: true
+  reject_old_samples_max_age: 168h