Add regex-matching support for labels.

There are four label-matching ops for selecting timeseries now:

- Equal: =
- NotEqual: !=
- RegexMatch: =~
- RegexNoMatch: !~

Instead of looking up labels by a simple clientmodel.LabelSet (basically
an equals op for every key/value pair in the set), timeseries
fingerprint selection is now done via a list of metric.LabelMatchers.

Change-Id: I510a83f761198e80946146770ebb64e4abc3bb96