anchor.diff:          '';
         };
         separator = mkOption {
  -        type = types.str;
  -        default = ";";
  +        type = types.nullOr types.str;
  +        default = null;
           description = ''
             Separator placed between concatenated source label values.
           '';
  
positive.code:         '';
        };
        regex = mkOption {
          type = types.str;
          default = "(.*)";
          description = ''
            Regular expression against which the extracted value is matched.
          '';
        };
        replacement = mkOption {
          type = types.str;
          default = "$1";
          description = ''
            Replacement value against which a regex replace is performed if the
            regular expression matches.
          '';
        };
        action = mkOption {
          type = types.enum ["replace" "keep" "drop"];
          default = "replace";
          description = ''
            Action to perform based on regex matching.
          '';
  
negative.code:         '';
        };
        scheme = mkOption {
          type = types.nullOr (types.enum ["http" "https"]);
          default = null;
          description = ''
            The URL scheme with which to fetch metrics from targets.
          '';
        };
        params = mkOption {
          type = types.nullOr (types.attrsOf (types.listOf types.str));
          default = null;
          description = ''
            Optional HTTP URL parameters.
          '';
  