Add support for OpenTracing

[xnslong]

This PR will add OpenTracing support to motan-go, and then we can collect the trace data to any compatible trace systems, such as zipkin.

What we should do if we wish to take use of this capability

1. Change configuration, add trace filter

...
motan-basicService:
example_basic_service:
  group: motan-go-example
  protocol: motan2
  registry: "direct-registry"
  filter: "accessLog,trace" # add 'trace' filter here
  serialization: simple
  nodeType: server
...

2. Set wanted Tracer

tracer, _ := zipkintracer.NewTracer(recorder, zipkintracer.WithLogger(logger))
opentracing.SetGlobalTracer(tracer)

for tracers, please referer to OpenTracing [Github]

3. [Optional] Set custom recording function

filter.TraceRecordingFunc = func(span opentracing.Span, data *filter.CallData) {
  // Do default recording
  filter.DefaultTraceRecordingFunc(span, data)
  // Do more recording
  span.SetTag("ca", motancore.LocalIP)
}

If not set, the filter.DefaultTraceRecordingFunc(span opentracing.Span, data *filter.CallData) is used directly, recording following information:

• service.type
• service.group
• peer.host
• peer.port
• error