Register copies the provided baseLabels

This ensures that you can pass the same base label set into multiple
Register() calls, e.g.:

    labels := map[string]string{"key": "value"}
    prometheus.Register("metric_1", "", labels, ...)
    prometheus.Register("metric_2", "", labels, ...)

Change-Id: I951e5c2ed7844c74eb3716d1bf07126ce558f266