tsweb: expose native Prometheus metrics in /debug/varz

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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](https://github.com/tailscale/corp/blob/a5b5d5167f1a2b780d313ec6a30e7b96dfa25b2d/cmd/tailcontrol/tailcontrol.go#L779)

Updates https://github.com/tailscale/corp/issues/10205

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

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