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>

cmd/tailscaled/tailscaled.go

@@ -49,7 +49,7 @@ import (
 	"tailscale.com/safesocket"
 	"tailscale.com/smallzstd"
 	"tailscale.com/syncs"
-	"tailscale.com/tsweb"
+	"tailscale.com/tsweb/varz"
 	"tailscale.com/types/flagtype"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/logid"
@@ -670,7 +670,7 @@ func newDebugMux() *http.ServeMux {
 
 func servePrometheusMetrics(w http.ResponseWriter, r *http.Request) {
 	w.Header().Set("Content-Type", "text/plain")
-	tsweb.VarzHandler(w, r)
+	varz.Handler(w, r)
 	clientmetric.WritePrometheusExpositionFormat(w)
 }