Add support for global MDC argument.

[gaoqiangz]

Global value for all threads output. Pattern argument: XG, gmdc

• {XG(node_id)} - node-win-554488715635
• {XG(nonexistent_key)(no mapping)} - no mapping

[estk]

@gaoqiangz Looks good, would you mind merging upstream first that way we're not pulling your git repo directly?

[gaoqiangz]

Please look again. But there's another PR that doesn't know when it will be merged: rust-log-mdc: Add global context support

[estk]

@sfackler any chance you can help us out?

[estk]

Looks like Steven was able to respond to the pr in rust-log-mdc. I'm not comfortable sidestepping that process and will therefore be closing this for now.