util/rands: add package with HexString func

We use it a number of places in different repos. Might as well make
one. Another use is coming.

Updates #cleanup

Change-Id: Ib7ce38de0db35af998171edee81ca875102349a4
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/localapi/localapi.go

@@ -7,8 +7,6 @@ package localapi
 import (
 	"bytes"
 	"context"
-	"crypto/rand"
-	"encoding/hex"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -49,6 +47,7 @@ import (
 	"tailscale.com/util/httpm"
 	"tailscale.com/util/mak"
 	"tailscale.com/util/osdiag"
+	"tailscale.com/util/rands"
 	"tailscale.com/version"
 )
 
@@ -118,12 +117,6 @@ var handler = map[string]localAPIHandler{
 	"query-feature":               (*Handler).serveQueryFeature,
 }
 
-func randHex(n int) string {
-	b := make([]byte, n)
-	rand.Read(b)
-	return hex.EncodeToString(b)
-}
-
 var (
 	// The clientmetrics package is stateful, but we want to expose a simple
 	// imperative API to local clients, so we need to keep track of
@@ -318,7 +311,7 @@ func (h *Handler) serveBugReport(w http.ResponseWriter, r *http.Request) {
 	defer h.b.TryFlushLogs() // kick off upload after bugreport's done logging
 
 	logMarker := func() string {
-		return fmt.Sprintf("BUG-%v-%v-%v", h.backendLogID, h.clock.Now().UTC().Format("20060102150405Z"), randHex(8))
+		return fmt.Sprintf("BUG-%v-%v-%v", h.backendLogID, h.clock.Now().UTC().Format("20060102150405Z"), rands.HexString(16))
 	}
 	if envknob.NoLogsNoSupport() {
 		logMarker = func() string { return "BUG-NO-LOGS-NO-SUPPORT-this-node-has-had-its-logging-disabled" }