proposal: x/sync/singleflight: Add Group.ForgetUnshared

[cuonglm]

After #44505, it's now easier to fix #31697.

I am able to re-unify "internal/singleflight" and "golang.org/x/sync/singleflight" APIs, the last missing piece is Group.ForgetUnshared, which is implemented in the former, but not the later.

I propose we add this method to golang.org/x/sync/singleflight.Group:

// ForgetUnshared tells the Group to forget about a key if it is not
// shared with any other goroutines. Future calls to Do for a forgotten key
// will call the function rather than waiting for an earlier call to complete.
// Returns whether the key was forgotten or unknown--that is, whether no
// other goroutines are waiting for the result.
func (g *Group) ForgetUnshared(key string) bool

to unblock #31697.

[cuonglm]

cc @bradfitz

[gopherbot]

Change https://go.dev/cl/424114 mentions this issue: singleflight: add Group.ForgetUnshare

[moriyoshi]

Is there any chance light will be shed on it again?