func (c *conn) Close() error {
	c.pool.decrRef()
	if c.once {
		return c.reset()
	}
	return nil
}
Close see Conn interface.
package pool
import (
	"google.golang.org/grpc"
)
type Conn interface {
	Value() *grpc.ClientConn
	Close() error
}
type conn struct {
	cc   *grpc.ClientConn
	pool *pool
	once bool
}
func (c *conn) Value() *grpc.ClientConn {
	return c.cc
}