Logs doesn't implement

[Mahito]

I tried to use this library for our service tracing with event logging, however it has not been implemented yet.

I found an implementation in zipkin-go-opentracing.
It seems to send logs that include timestamp and fields converted to strings in Annotations.
I could not find any other implementation, but since it is the official implementation of openzipkin.

I tried a similar implementation.

Sample code and results image using this function are as follows.

require './lib/zipkin/tracer'

OpenTracing.global_tracer = Zipkin::Tracer.build(url: 'http://localhost:9411', service_name: 'echo')

span = OpenTracing.start_span('span name')
span.log_kv({'event': 'test'})
span.log_kv({'key1': 'val1', 'key2': 'val2'})
span.finish
OpenTracing.global_tracer.stop```

[indrekj]

From Java implementation: https://github.com/openzipkin-contrib/brave-opentracing/blob/master/src/main/java/brave/opentracing/BraveSpan.java#L160-L169.

I will look into this later this week.

[indrekj]

Please see #11 and let me know if that looks okay.