feat: add Prometheus collector for DERP server expvar metrics (#22583)

This PR does three things:
- Exports derp expvars to the pprof endpoint
- Exports the expvar metrics as prometheus metrics in both coderd and
wsproxy
- Updates our tailscale to a fix I also had to make to avoid a data race
condition

I generated this with mux but I also manually tested that the metrics
were getting properly emitted

go.sum

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@@ -1525,6 +1529,8 @@ kernel.org/pub/linux/libs/security/libcap/psx v1.2.77 h1:Z06sMOzc0GNCwp6efaVrIrz
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