tsweb: move varz handler(s) into separate modules

This splits Prometheus metric handlers exposed by tsweb into two
modules:
- `varz.Handler` exposes Prometheus metrics generated by our expvar
  converter;
- `promvarz.Handler` combines our expvar-converted metrics and native
  Prometheus metrics.

By default, tsweb will use the promvarz handler, however users can keep
using only the expvar converter. Specifically, `tailscaled` now uses
`varz.Handler` explicitly, which avoids a dependency on the
(heavyweight) Prometheus client.

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

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

tsweb/debug.go

@@ -14,6 +14,8 @@ import (
 	"os"
 	"runtime"
 
+	"tailscale.com/tsweb/promvarz"
+	"tailscale.com/tsweb/varz"
 	"tailscale.com/version"
 )
 
@@ -51,10 +53,10 @@ func Debugger(mux *http.ServeMux) *DebugHandler {
 	// index page. The /pprof/ index already covers it.
 	mux.Handle("/debug/pprof/profile", http.HandlerFunc(pprof.Profile))
 
-	ret.KVFunc("Uptime", func() any { return Uptime() })
+	ret.KVFunc("Uptime", func() any { return varz.Uptime() })
 	ret.KV("Version", version.Long())
 	ret.Handle("vars", "Metrics (Go)", expvar.Handler())
-	ret.Handle("varz", "Metrics (Prometheus)", http.HandlerFunc(CombinedVarzHandler))
+	ret.Handle("varz", "Metrics (Prometheus)", http.HandlerFunc(promvarz.Handler))
 	ret.Handle("pprof/", "pprof", http.HandlerFunc(pprof.Index))
 	ret.URL("/debug/pprof/goroutine?debug=1", "Goroutines (collapsed)")
 	ret.URL("/debug/pprof/goroutine?debug=2", "Goroutines (full)")