Miscellaneous changes

Tom:

A collection of miscellaneous changes (each in their own commit) I had laying around that I think might be useful, but I hadn't yet taken the time to turn them into individuals MRs.

Flo:
All good ideas, albeit with some small slips that I fixed (using many more commits than would be required I am afraid).

nixos/modules/monitoring/exporters/node.nix

@@ -15,6 +15,11 @@ let
 
 in {
   config.services.prometheus.exporters.node = {
+    textfiles-directory = lib.mkOption {
+      type = lib.types.str;
+      description = "Directory used by the textfiles collector.";
+      default = "/run/prometheus-node-exporter/tahoe.prom";
+    };
     enable = true;
     openFirewall = true;
     firewallFilter = "-i monitoringvpn -p tcp -m tcp --dport 9100";
@@ -36,7 +41,7 @@ in {
       "stat"
       "systemd"
       "textfile"
-      "textfile.directory /run/prometheus-node-exporter"
+      "textfile.directory ${cfg.textfiles-directory}"
       #"thermal_zone" # not in nixpkgs 19.09
       "time"
       #"udp_queues" # not in nixpkgs 19.09