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: https://github.com/openzipkin/zipkin/issues/939

Related to https://github.com/microservices-demo/orders/issues/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. :x: https://github.com/go-kit/kit/issues/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 :x: https://github.com/openzipkin/zipkin-go-opentracing/issues/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 20be7a4adb39a2478f04382ddbd5cffa8db14f08 on gfads:enhancement/zipkin-tracing into b5ec77ee831c0ae1c1411fce4bdd73e67a87c033 on microservices-demo:master.

[embs]

I've removed config for sharing span as Zipkin is moving away from client/server shared spans: https://github.com/openzipkin/zipkin/issues/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 52923f977ea84c5f1253b8903372e4965cf894c6 on gfads:enhancement/zipkin-tracing into b5ec77ee831c0ae1c1411fce4bdd73e67a87c033 on microservices-demo:master.

[coveralls]

Coverage remained the same at 23.465% when pulling 10b11fe97e46e45b02a26481fe2b9f7edc042ebc on gfads:enhancement/zipkin-tracing into 013b519efbe9d848cf16969db7d1d15a1e58c28a on microservices-demo:master.