Zipkin exporter must support Zipkin v2 Protobuf format

[codeboten]

As per spec: https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/sdk_exporters/zipkin.md#considerations-for-legacy-v1-format

[robwknox]

@codeboten i'm taking a stab at this if you care to assign me.