Merge pull request '#347 add Sort Labels' (#47) from kgorbunov/micro-tracer-opentracing:#347-v3 into v3

Reviewed-on: #47

opentracing.go

@@ -10,6 +10,7 @@ import (
 	"go.opentelemetry.io/otel/attribute"
 	"go.unistack.org/micro/v3/metadata"
 	"go.unistack.org/micro/v3/tracer"
+	"go.unistack.org/micro/v3/util/sort"
 )
 
 var _ tracer.Tracer = &otTracer{}
@@ -183,6 +184,8 @@ func (os *otSpan) Finish(opts ...tracer.SpanOption) {
 		options.Labels = options.Labels[:len(options.Labels)-1]
 	}
 
+	os.opts.Labels = sort.Uniq(os.opts.Labels)
+
 	l := len(options.Labels)
 	for idx := 0; idx < l; idx++ {
 		switch lt := options.Labels[idx].(type) {