metrics: add histogram support

Add initial histogram support.

Updates tailscale/corp#8641

Signed-off-by: Maisem Ali <maisem@tailscale.com>

tsweb/varz/varz.go

@@ -191,6 +191,8 @@ func writePromExpVar(w io.Writer, prefix string, kv expvar.KeyValue) {
 		v.Do(func(kv expvar.KeyValue) {
 			fmt.Fprintf(w, "%s{%s=%q} %v\n", name, cmpx.Or(v.Label, "label"), kv.Key, kv.Value)
 		})
+	case *metrics.Histogram:
+		v.PromExport(w, name)
 	case *expvar.Map:
 		if label != "" && typ != "" {
 			fmt.Fprintf(w, "# TYPE %s %s\n", name, typ)