tsweb/varz: flesh out munging of expvar keys into valid Prometheus metrics

From a problem we hit with how badger registers expvars; it broke
trunkd's exported metrics.

Updates tailscale/corp#1297

Change-Id: I42e1552e25f734c6f521b6e993d57a82849464b2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tsweb/varz/varz_test.go

@@ -43,6 +43,24 @@ func TestVarzHandler(t *testing.T) {
 			new(expvar.Int),
 			"# TYPE foo_bar counter\nfoo_bar 0\n",
 		},
+		{
+			"slash_in_metric_name",
+			"counter_foo/bar",
+			new(expvar.Int),
+			"# TYPE foo_bar counter\nfoo_bar 0\n",
+		},
+		{
+			"metric_name_start_digit",
+			"0abc",
+			new(expvar.Int),
+			"# TYPE _0abc counter\n_0abc 0\n",
+		},
+		{
+			"metric_name_have_bogus_bytes",
+			"abc\x10defügh",
+			new(expvar.Int),
+			"# TYPE abcdefgh counter\nabcdefgh 0\n",
+		},
 		{
 			"int_with_type_counter",
 			"counter_foo",