tracer: add Flush method

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

tracer/noop.go

@@ -4,6 +4,8 @@ import (
 	"context"
 )
 
+var _ Tracer = (*noopTracer)(nil)
+
 type noopTracer struct {
 	opts Options
 }
@@ -21,6 +23,10 @@ func (t *noopTracer) Start(ctx context.Context, name string, opts ...SpanOption)
 	return NewSpanContext(ctx, span), span
 }
 
+func (t *noopTracer) Flush(ctx context.Context) error {
+	return nil
+}
+
 func (t *noopTracer) Init(opts ...Option) error {
 	for _, o := range opts {
 		o(&t.opts)