Consider requiring a feature to be implemented in a few libraries before making the spec final

[sebright2]

See https://github.com/census-instrumentation/opencensus-specs/issues/195#issuecomment-430091302.

We could update https://github.com/census-instrumentation/opencensus-specs/blob/a6ad82f4ee21038c33d5282ebb4979d08808e6b5/Process.md#step-3-specs-pull-request to require each new feature to be merged with a "draft" label. Then, after the feature has been implemented in a few libraries and has been shown to work well, we could remove the "draft" label.

/cc @adriancole @bogdandrutu

[bogdandrutu]

+1 I think this is a good idea.