fix metrics

[yiguo-wish]

panic: inconsistent label cardinality: expected 4 label values but got 3 in []string{"ListQueues", "prod", ""}
goroutine 7 [running]:
github.com/wish/qproxy/vendor/github.com/prometheus/client_golang/prometheus.

[jazhang-wish]

@yiguo-wish is url being used as tag here? if so, it doesn't seem to be good since the cardinality may be large.

[yiguo-wish]

It should be fine, that metrics only triggers when we have malformed queue names, which is quite rare.
I'm expecting even that metrics are triggered, it should come from one url
Usually that field is queue name, but in listqueue function, we don't have that info