func WithRetry(fn func() Retryer) CallOption {
	return retryerOption(fn)
}
WithRetry sets CallSettings.Retry to fn.
package gax
import (
	"math/rand"
	"time"
	"google.golang.org/grpc"
	"google.golang.org/grpc/codes"
	"google.golang.org/grpc/status"
)
type CallOption interface {
	Resolve(cs *CallSettings)
}
type Retryer interface {
	Retry(err error) (pause time.Duration, shouldRetry bool)
}
type retryerOption func() Retryer
func (o retryerOption) Resolve(s *CallSettings) {
	s.Retry = o
}