Add hyphen to allowed characters for dimension keys
Add hyphen to allowed characters for dimension keys

According to [the documentation](https://developers.signalfx.com/v2/reference#section-criteria-for-metric-and-dimension-names-and-values) hyphens are allowed in dimension keys, but the protobof encoding for HTTPClient disallows them. This modifies the allowed characters to include `-`.
signalfx/golib,signalfx/golib