PoE collector (#54)

Thanks for the PR.

* PoE collector
* Correcting mistake in comment for WithPOE function

collector/collector.go

@@ -84,6 +84,13 @@ func WithDHCPv6() Option {
 	}
 }
 
+// WithPOE enables PoE metrics
+func WithPOE() Option {
+	return func(c *collector) {
+		c.collectors = append(c.collectors, newPOECollector())
+	}
+}
+
 // WithPools enables IP(v6) pool metrics
 func WithPools() Option {
 	return func(c *collector) {