mocktracer/mocktracer.go
func (t *MockTracer) Extract(format interface{}, carrier interface{}) (opentracing.SpanContext, error) {
	extractor, ok := t.extractors[format]
	if !ok {
		return nil, opentracing.ErrUnsupportedFormat
	}
	return extractor.Extract(carrier)
}
func (t *MockTracer) Extract(format interface{}, carrier interface{}) (opentracing.SpanContext, error) {
	extractor, ok := t.extractors[format]
	if !ok {
		return nil, opentracing.ErrUnsupportedFormat
	}
	return extractor.Extract(carrier)
}
// Extract belongs to the Tracer interface.