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>

tstest/integration/tailscaled_deps_test_darwin.go

@@ -36,7 +36,7 @@ import (
 	_ "tailscale.com/ssh/tailssh"
 	_ "tailscale.com/syncs"
 	_ "tailscale.com/tailcfg"
-	_ "tailscale.com/tsweb"
+	_ "tailscale.com/tsweb/varz"
 	_ "tailscale.com/types/flagtype"
 	_ "tailscale.com/types/key"
 	_ "tailscale.com/types/logger"

tstest/integration/tailscaled_deps_test_freebsd.go

@@ -36,7 +36,7 @@ import (
 	_ "tailscale.com/ssh/tailssh"
 	_ "tailscale.com/syncs"
 	_ "tailscale.com/tailcfg"
-	_ "tailscale.com/tsweb"
+	_ "tailscale.com/tsweb/varz"
 	_ "tailscale.com/types/flagtype"
 	_ "tailscale.com/types/key"
 	_ "tailscale.com/types/logger"

tstest/integration/tailscaled_deps_test_linux.go

@@ -36,7 +36,7 @@ import (
 	_ "tailscale.com/ssh/tailssh"
 	_ "tailscale.com/syncs"
 	_ "tailscale.com/tailcfg"
-	_ "tailscale.com/tsweb"
+	_ "tailscale.com/tsweb/varz"
 	_ "tailscale.com/types/flagtype"
 	_ "tailscale.com/types/key"
 	_ "tailscale.com/types/logger"

tstest/integration/tailscaled_deps_test_openbsd.go

@@ -36,7 +36,7 @@ import (
 	_ "tailscale.com/ssh/tailssh"
 	_ "tailscale.com/syncs"
 	_ "tailscale.com/tailcfg"
-	_ "tailscale.com/tsweb"
+	_ "tailscale.com/tsweb/varz"
 	_ "tailscale.com/types/flagtype"
 	_ "tailscale.com/types/key"
 	_ "tailscale.com/types/logger"

tstest/integration/tailscaled_deps_test_windows.go

@@ -43,7 +43,7 @@ import (
 	_ "tailscale.com/smallzstd"
 	_ "tailscale.com/syncs"
 	_ "tailscale.com/tailcfg"
-	_ "tailscale.com/tsweb"
+	_ "tailscale.com/tsweb/varz"
 	_ "tailscale.com/types/flagtype"
 	_ "tailscale.com/types/key"
 	_ "tailscale.com/types/logger"