Add tracing support to the StackDriver adapter.

[geeknoid]

Have the StackDriver adapter implement the template designed in #1509

[semistrict]

I am actively working on this here: https://github.com/ramonza/istio/tree/stackdriver-trace
@douglas-reid would you like to assign this to me?

[douglas-reid]

@Ramonza sure. Do you want to open an issue in istio/istio and track it there? This repo is just a legacy code holder at this point.