0.9.3: Custom label support

Each of the metric methods now allow adding extra labels:

testCounter.Increment(
    net.ParseIP("127.0.0.1"),
    metrics.Label("foo", "bar"),
    metrics.Label("somelabel","somevalue")
)

The following rules apply and will cause a panic if violated:

• Label names and values cannot be empty.
• The country label name is reserved for GeoIP usage.