Support underscores in metric name components

[matthiasr]

Break-out from #80:

The exporter's metric parsing currently fails if any component of the Graphite metric name contains underscores, unless there is a mapping that constructs a valid Prometheus name.

Even without a matching mapping, the exporter should handle these metric names and translate them to valid Prometheus metric names. Existing metric names for metrics without underscores should not change when doing this.

[glightfoot]

@xkilian can you provide the mapping config and some more example metrics that don't work? I threw together some tests on master and it appears to work fine

[xkilian]

The case here, is for a metric whose name is structured like: hostname_one.source.detail_modifier.type
If there is no matching mapping rule and it tries to do auto-mapping to a prometheus metric.
If the mapping config drops all unmatched metrics by default, then this does not apply. I will get something together.