feat: export numG and qSize

[guilhem]

This permit to create a group without using WithContext.
As cancel is nil, errgroup will not send a context close at first error.

[neilotoole]

@guilhem Apologies for the egregious delay in responding to you. Thank you for the PR.

One of the reasons that numG and qSize are unexported is because we don't want those field values to change during the lifetime of the Group... or at a minimum, updating those fields should be done atomically.

As cancel is nil, errgroup will not send a context close at first error.

Is this not desirable?

This permit to create a group without using WithContext.

Can you elaborate some more on the issue you're facing that this PR resolves? Thank you.