nixosTests.prometheus-exporters.bind: check bind_up metric

Check for 'bind_up 1' instead of 'bind_query_recursions_total 0'.

The bind_query_recursions_total metric is only emitted when there have
been recursive queries. In a fresh BIND instance with no traffic, this
counter doesn't exist. The bind_up metric reliably indicates that the
exporter successfully connected to BIND's statistics channel.

nixos/tests/prometheus-exporters.nix

@@ -108,7 +108,7 @@ let
           wait_for_unit("prometheus-bind-exporter.service")
           wait_for_open_port(9119)
           succeed(
-              "curl -sSf http://localhost:9119/metrics | grep 'bind_query_recursions_total 0'"
+              "curl -sSf http://localhost:9119/metrics | grep 'bind_up 1'"
           )
         '';
       };