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

scripts/metricsdocgen/scanner/scanner.go

@@ -30,6 +30,7 @@ var scanDirs = []string{
 	"coderd",
 	"enterprise",
 	"provisionerd",
+	"tailnet",
 }
 
 // skipPaths lists files that should be excluded from scanning. Their metrics