Initialise counters w/o using Set.

CounterVec#Set was present in v0.8.0 but has been removed in 0.9.0-pre1.
Simply calling the counter with a label seems to initialise it.
If not calling it, then the metric/label is not exposed until actually incremented, which is probably not the desired behaviour -- i.e. people would most likely still want to know that the number of debug/info/warning/error log messages is zero, and to confirm that their monitoring is not broken.

promrus.go

@@ -22,7 +22,7 @@ func NewPrometheusHook() (*PrometheusHook, error) {
 	}, []string{"level"})
 	// Initialise counters for all supported levels:
 	for _, level := range supportedLevels {
-		counterVec.WithLabelValues(level.String()).Set(0)
+		counterVec.WithLabelValues(level.String())
 	}
 	// Try to register the counter vector:
 	err := prometheus.Register(counterVec)