Add option for LightStep tracing to ProxyConfig

[JulianGriggs]

This PR adds configuration for a LightStep tracing backend to the ProxyConfig proto.

A WIP/POC implementation can be found here. When this PR gets finalized I will update and submit the implementation for review.

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[JulianGriggs]

I signed it!

[rshriram]

Thanks for doing this! It would be good to consolidate the tracing config so that we pave the way for other tracers as well.

[rshriram]

Mark the zipkin address as deprecated. And create a tracing message that is a one-of between Zipkin/lightstep. something like

message Tracing {
 message Zipkin {
   address string
 }
 message LightStep {
  ...
 }
  one_of {
    Zipkin zipkin;
    Lightstep lightstep;
 }
}
Tracing tracing

The proxyconfig is internal data structure. We formalized it here for some documentation purposes. So, there is no backward compatibility issues here. It merely translates to the set of CLI args that we pass to the pilot agent [which you will have to update anyway to enable lightstep].

[googlebot]

CLAs look good, thanks!

[JulianGriggs]

@rshriram Thanks for the review! I created a consolidated Tracing message as you described. Question: do you think it should be a top-level message or should it be nested within ProxyConfig? I currently have it as a top level message but am happy to change it if you think it would be better off nested.

[JulianGriggs]

/assign @rshriram

[JulianGriggs]

pinging @frankbu @hklai @rshriram for review :)

[rshriram]

LGTM
I hope you also have a PR in istio/istio for this?

[rshriram]

/ok-to-test