Previously, the api was evaluating this regex to determine if the label
name was valid or not:
14bac55a99/model/labels.go (L94)
However, I believe that the `IsValid()` function is what ought to be
used in the brave new utf8 era.
**Before**
```
$ curl localhost:9090/api/v1/label/host.name/values
{"status":"error","errorType":"bad_data","error":"invalid label name: \"host.name\""}
```
**After**
```
$ curl localhost:9090/api/v1/label/host.name/values
{"status":"success","data":["localhost"]}
```
It's very likely that I'm missing something here or you were already
planning to do this at some point but I just encountered this issue and
figured I'd give it a go.
Signed-off-by: Owen Williams <owen.williams@grafana.com>
88675710f9