code: func (t *Tracer) StartSpanFromContext(ctx context.Context, operationName string) (tracing.Span, context.Context) {
  	var opts []opentracing.StartSpanOption
  	if parent := opentracing.SpanFromContext(ctx); parent != nil {
  		opts = append(opts, opentracing.ChildOf(parent.Context()))
  	}
  	span := t.tracer.StartSpan(operationName, opts...)
  	return span, opentracing.ContextWithSpan(ctx, span)
  }
docs: // StartSpanFromContext returns a new child span and context from a given context.