func Int63n(n int64) int64 {
	mu.Lock()
	res := r.Int63n(n)
	mu.Unlock()
	return res
}
Int63n implements rand.Int63n on the grpcrand global source.
package grpcrand
import (
	"math/rand"
	"sync"
	"time"
)
var (
	r  = rand.New(rand.NewSource(time.Now().UnixNano()))
	mu sync.Mutex
)