Opencensus exporters should populate service_name from Span Resource

[lzchen]

Fixes #1714

[srikanthccv]

We should probably get rid of this logic in exporter(s) init.

        tracer_provider = trace.get_tracer_provider()
        service_name = (
            tracer_provider.resource.attributes[SERVICE_NAME]
            if getattr(tracer_provider, "resource", None)
            else Resource.create().attributes.get(SERVICE_NAME)
        )