Add support for trace-context headers

[chadbrad]

Please add trace-context header support :)

Please see this doc for the problem statement and specifics: https://docs.google.com/document/d/1ukvnSoXkavcojlVXpCRzep8PS_guXly_NbbhHIDgbFY/edit

[ocelotl]

I'm starting to work on this issue since #80 is under review now.