nixos/tests/prometheus-exporters/knot: update for new exporter version

The knot_server_zone_count metric does not exist anymore, and the next
best thing to watch for is the zone serial, that we define ourselves.

The serial is a number and displayed in the scientific notation, i.e.

    >>> machine.succeed('curl localhost:9433/metrics|grep 019 >&2')
    [...]
    knot # knot_zone_serial{zone="test."} 2.019031301e+09

nixos/tests/prometheus-exporters.nix

@@ -512,7 +512,7 @@ let
         wait_for_unit("knot.service")
         wait_for_unit("prometheus-knot-exporter.service")
         wait_for_open_port(9433)
-        succeed("curl -sSf 'localhost:9433' | grep 'knot_server_zone_count 1.0'")
+        succeed("curl -sSf 'localhost:9433' | grep '2\.019031301'")
       '';
     };