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, ...)
Merge pull request #29 from prometheus/fix/copy-base-labels-in-register

Register copies the provided baseLabels