prober: migrate to Prometheus metric library

This provides an example of using native Prometheus metrics with tsweb.

Prober library seems to be the only user of PrometheusVar, so I am
removing support for it in tsweb.

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

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

tsweb/tsweb_test.go

@@ -9,7 +9,6 @@ import (
 	"errors"
 	"expvar"
 	"fmt"
-	"io"
 	"net"
 	"net/http"
 	"net/http/httptest"
@@ -570,12 +569,6 @@ foo_totalY 4
 			expvar.Func(func() any { return 123 }),
 			"num_goroutines 123\n",
 		},
-		{
-			"var_that_exports_itself",
-			"custom_var",
-			promWriter{},
-			"custom_var_value 42\n",
-		},
 		{
 			"string_version_var",
 			"foo_version",
@@ -694,16 +687,6 @@ func (a expvarAdapter2) PrometheusMetricsReflectRoot() any {
 	return a.st
 }
 
-type promWriter struct{}
-
-func (promWriter) WritePrometheus(w io.Writer, prefix string) {
-	fmt.Fprintf(w, "%s_value 42\n", prefix)
-}
-
-func (promWriter) String() string {
-	return ""
-}
-
 func TestAcceptsEncoding(t *testing.T) {
 	tests := []struct {
 		in, enc string