Get Span from ctx

[matianjun1]

Sometime I need spanFromContext

such as go-redis-hook

type Hook interface {
    BeforeProcess(ctx context.Context, cmd Cmder) (context.Context, error)
    AfterProcess(ctx context.Context, cmd Cmder) error
}

only ctx can be propagation, I cant do span.End further

Maybe there is a easy way to solve, set ctxKeyInstance to be public. Then I can use this key to get activeSpan from ctx and do (.Span) to get a Span

[wu-sheng]

Typically, this should be called ctx#activeSpan. We are welcome the contribution.