v3 (#404)

## Pull Request template
Please, go through these steps before clicking submit on this PR.

1. Give a descriptive title to your PR.
2. Provide a description of your changes.
3. Make sure you have some relevant tests.
4. Put `closes #XXXX` in your comment to auto-close the issue that your PR fixes (if applicable).

**PLEASE REMOVE THIS TEMPLATE BEFORE SUBMITTING**

Reviewed-on: #404
Co-authored-by: Evstigneev Denis <danteevstigneev@yandex.ru>
Co-committed-by: Evstigneev Denis <danteevstigneev@yandex.ru>

tracer/tracer.go

@@ -51,6 +51,8 @@ type Tracer interface {
 	// Extract(ctx context.Context)
 	// Flush flushes spans
 	Flush(ctx context.Context) error
+	// Enabled returns tracer status
+	Enabled() bool
 }
 
 type Span interface {