Improved flare-stats output and fixed invalid digit problem

when items == 1234567890
```
before:
hostname:port               state   role partition balance    items conn behind hit size uptime version   qps qps-r qps-w
localhost:12121            active  proxy        -1       0 12345678    2      0   -    0     2h   1.1.0     0     0     0

after:
hostname:port   state  role  partition balance items      conn behind hit size uptime version qps qps-r qps-w
localhost:12121 active proxy        -1       0 1234567890    2      0   -    0     2h   1.1.0   0     0     0
```