Title: Add support for sending data in Zipkin format

Description:
It would be really helpful to support sending data in zipkin v2 json for reasons of efficiency and reduced tech debt in trace data pipelines. This could go alongside the existing functionality. While proto3 is even more efficient, it could be optional as most don't use that.

Zipkin's data format has been historically criticized for its heft. Through community effort, last August 2017 we formalized a compact v2 json, accepted on all transports including http, kafka, rabbit, etc. Later, we introduced a proto3 encoding of the same, also accepted on all transports.

Between last August and now, this format has become the primary and preferred format, especially to those trying to work with the data. For example, at a recent meeting we've found that envoy is a key piece of the network that still emits the v1 format, requiring "rosetta stone" style proxies.

This is fine in pure zipkin installs as the server reads all historical formats, but is limiting for those using different pipelines. A switch to v2 would also work with zipkin clones such as jaeger who also support that format.

original spec for zipkin and envoy
json v2 encoding
proto3 encoding
trace data pipeline meeting
jaeger's v2 support