all: use cmpx.Or where it made sense

I left a few out where writing it explicitly was better
for various reasons.

Updates #8296

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

tsweb/tsweb.go

@@ -27,6 +27,7 @@ import (
 	"tailscale.com/net/tsaddr"
 	"tailscale.com/tsweb/varz"
 	"tailscale.com/types/logger"
+	"tailscale.com/util/cmpx"
 	"tailscale.com/util/vizerror"
 )
 
@@ -144,10 +145,7 @@ func (h Port80Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		// Redirect authorized user to the debug handler.
 		path = "/debug/"
 	}
-	host := h.FQDN
-	if host == "" {
-		host = r.Host
-	}
+	host := cmpx.Or(h.FQDN, r.Host)
 	target := "https://" + host + path
 	http.Redirect(w, r, target, http.StatusFound)
 }

tsweb/varz/varz.go

@@ -18,6 +18,7 @@ import (
 	"time"
 
 	"tailscale.com/metrics"
+	"tailscale.com/util/cmpx"
 	"tailscale.com/version"
 )
 
@@ -96,16 +97,10 @@ func writePromExpVar(w io.Writer, prefix string, kv expvar.KeyValue) {
 
 	switch v := kv.Value.(type) {
 	case *expvar.Int:
-		if typ == "" {
-			typ = "counter"
-		}
-		fmt.Fprintf(w, "# TYPE %s %s\n%s %v\n", name, typ, name, v.Value())
+		fmt.Fprintf(w, "# TYPE %s %s\n%s %v\n", name, cmpx.Or(typ, "counter"), name, v.Value())
 		return
 	case *expvar.Float:
-		if typ == "" {
-			typ = "gauge"
-		}
-		fmt.Fprintf(w, "# TYPE %s %s\n%s %v\n", name, typ, name, v.Value())
+		fmt.Fprintf(w, "# TYPE %s %s\n%s %v\n", name, cmpx.Or(typ, "gauge"), name, v.Value())
 		return
 	case *metrics.Set:
 		v.Do(func(kv expvar.KeyValue) {