Decide what we want about macro_metavar_expr

[traviscross]

The author of the stabilization PR for macro_metavar_expr has apparently lost interest in further pursuing it:

• Stabilize count, ignore, index, and len (macro_metavar_expr) #122808

We've reopened it for now, but probably it'll need a new PR regardless. To clear the way for that, probably we should decide what it is that we want to see here. Let's use this issue to track our answering of that.

@rustbot labels +T-lang +C-discussion

cc @rust-lang/lang @rust-lang/wg-macros