Add an example of integration of OBI and Injector in opentelemetry-demo

[atoulme]

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Describe the issue you're reporting

As discussed in 11/17 SIG: add an example of integration of OBI and injector with the opentelemetry demo, showing a Go service with OBI interact with a Java service with the injector, and how both can stitch traces together.

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[atoulme]

@rafaelroquetto