pushgateway support

[matthewmueller]

Would be cool to be able support prometheus's pushgateway, then you can send metrics from serverless functions and even the browser!

[matthewmueller]

FWIW, I got started on this work here: https://github.com/matthewmueller/stats/tree/pushgateway. I'm still familiarizing myself with this library, so if you see anything that's off, let me know!

Looks like prometheus doesn't export pushgateway for programmatic use, so this will require either changes to prometheus or adding the server in here (probably easier since Handler already exists).

• server implementation of pushgateway
• client implementation of pushgateway