Add const labels to isolate metrics

[gunli]

Add const labels to isolate metrics.

Use case:
I am writing a logs reporter that pushing logs into different kafka topics, and I used sarama as the kafkaclient SDK, sarama used go metrics to collect metrics and I would like to export the metrics to prometheus, so I used go-metrics-prometheus to do that for me, but the original version of go-metrics-prometheus can isoltate metrics just by namespace/subsystem, and if I set different namespace/subsystem to different topics, it will produce different metrics names, which is not good for configuring metrics panels in grafana.

Now, I add a const label for each topic, the metrics names keep the same for each topics, but can be ioslated by the label values, I think it is useful for the guys who have the same requirements as me.