promql/engine.go
func (q *query) Exec(ctx context.Context) *Result {
	if span := opentracing.SpanFromContext(ctx); span != nil {
		span.SetTag(queryTag, q.stmt.String())
	}

	res, warnings, err := q.ng.exec(ctx, q)
	return &Result{Err: err, Value: res, Warnings: warnings}
}
func (q *query) Exec(ctx context.Context) *Result {
	if span := opentracing.SpanFromContext(ctx); span != nil {
		span.SetTag(queryTag, q.stmt.String())
	}

	res, warnings, err := q.ng.exec(ctx, q)
	return &Result{Err: err, Value: res, Warnings: warnings}
}
// Exec implements the Query interface.