The handler will expose built-in process and Go metrics by default,
which currently duplicate some of the expvar-proxied metrics
(`goroutines` vs `go_goroutines`, `memstats` vs `go_memstats`), but as
long as their names are different, Prometheus server will just scrape
both.
This will change /debug/varz behaviour for most tsweb binaries, but
notably not for control, which configures a `tsweb.VarzHandler`
[explicitly](a5b5d5167f/cmd/tailcontrol/tailcontrol.go (L779))
Updates https://github.com/tailscale/corp/issues/10205
Signed-off-by: Anton Tolchanov <anton@tailscale.com>
11e6247d2a