merge develop into production

• Grid configs: clean up config imports (!269 (merged))
• Grafana: Revamp network errors panel, add (experimental) PSI panels (!268 (merged))
• Miscellaneous changes (!262 (merged))
• define groups for tahoe nodes (!265 (merged))
• Don't display the port number in Grafana dash tooltips (!267 (merged))
• nixpkgs update 2022-02-21 (!266 (merged))
• Grafana 8: Migrate Dashboards from Graph to TimeSeries panels (!263 (merged))

morph/grid/local/grid.nix

 let
-  pkgs = import <nixpkgs> { };
-
   gridlib = import ../../lib;
-  grid-config = pkgs.lib.trivial.importJSON ./config.json;
+  grid-config = builtins.fromJSON (builtins.readFile ./config.json);
 
   ssh-users = let
     ssh-users-file = ./public-keys/users.nix;
@@ -131,7 +129,7 @@ let
   };
 
   # TBD: derive these automatically:
-  paymentExporterTargets = [ "payments" ];
+  paymentExporterTargets = [ "payments.monitoringvpn" ];
   blackboxExporterHttpsTargets = [
     # "https://private.storage/"
     # "https://payments.private.storage/"