Add local IP address to Zipkin tracing

[embs]

Also reuse client span to be consistent with how interactions between
Java services are traced. UPDATE: removed as Zipkin is moving away from client/server shared spans: openzipkin/zipkin#939

Related to microservices-demo/orders#48

An example of how these changes affect traces:

Before modification

Complete trace: original.zip

After modification

Complete trace: after-modification.zip

The highlighted frames in second image refers to:

1. Go service overriding operation name in span. ❌ Do not always override operation name when tracing endpoints go-kit/kit#615
2. Go service adding SR and SS annotations to the span. ✅
3. Go service adding binnary annotations which are also added by Java (caller) service ❌ Activating ClientServerSameSpan resulted in duplicated binnary annotations openzipkin-contrib/zipkin-go-opentracing#81

1 and 3 are not huge problems but I'm working on them in order to achieve consistence with how Java <-> Java services interactions are traced.

[coveralls]

Coverage remained the same at 23.465% when pulling 20be7a4 on gfads:enhancement/zipkin-tracing into b5ec77e on microservices-demo:master.

[embs]

I've removed config for sharing span as Zipkin is moving away from client/server shared spans: openzipkin/zipkin#939

So final format looks like this:

Client span

Server span

Complete trace: trace.zip

[coveralls]

Coverage remained the same at 23.465% when pulling 52923f9 on gfads:enhancement/zipkin-tracing into b5ec77e on microservices-demo:master.

[coveralls]

Coverage remained the same at 23.465% when pulling 10b11fe on gfads:enhancement/zipkin-tracing into 013b519 on microservices-demo:master.

[vlal-zz]

LGTM