It looks like such a daemon already exists (varnishstatsd), though I wasn't able to find its data anywhere in Graphite.
The main thing I'd like to start tracking is backend response time per data centre and varnish cluster (e.g. eqiad/text). We'll need to make sure to only count new backend requests (and not re-read the header from cached responses).
I'd be nice if we could also break down the text cluster slightly. e.g. varnish.esams.text.all but also varnish.esams.text.load_php for load.php requests. Alternatively, instead of hardcoding entry points in this daemon we could add them to varnishrls instead (and others where we specifically want them).