tracer: add labels method

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

tracer/noop.go

@@ -60,6 +60,10 @@ func (s *noopSpan) SetLabels(labels ...interface{}) {
 	s.opts.Labels = labels
 }
 
+func (s *noopSpan) AddLabels(labels ...interface{}) {
+	s.opts.Labels = append(s.opts.Labels, labels...)
+}
+
 // NewTracer returns new memory tracer
 func NewTracer(opts ...Option) Tracer {
 	return &noopTracer{

tracer/tracer.go

@@ -31,4 +31,6 @@ type Span interface {
 	SetName(name string)
 	// SetLabels set the span labels
 	SetLabels(labels ...interface{})
+	// AddLabels append the span labels
+	AddLabels(labels ...interface{})
 }