prepare statement count gives negative value

[YangKeao]

Use the following sysbench lua script:

function thread_init()
  drv = sysbench.sql.driver()
  con = drv:connect()

  stmt = con:prepare(string.format([[
        INSERT INTO t(j, v) VALUES (?, '%s')
       ]], rand_varstr(35840, 35840)))
end

function thread_done()
  stmt:close()
  con:disconnect()
end

After stopping and restarting this benchmark several times, the grafana panel of prepare statement count turns into negative value:

[YangKeao]

Fixed by #49073