tsweb: set charset when exporting vars to prometheus

Appears to be safe based on: https://github.com/prometheus/common/pull/119

Signed-off-by: Tom DNetto <tom@tailscale.com>
Updates: corp#17075

tsweb/varz/varz.go

@@ -240,7 +240,7 @@ type sortedKVs struct {
 //
 // This will evolve over time, or perhaps be replaced.
 func Handler(w http.ResponseWriter, r *http.Request) {
-	w.Header().Set("Content-Type", "text/plain; version=0.0.4")
+	w.Header().Set("Content-Type", "text/plain; version=0.0.4; charset=utf-8")
 
 	s := sortedKVsPool.Get().(*sortedKVs)
 	defer sortedKVsPool.Put(s)