export eos quota information

Expose metrics from eos quota subcomand. 
```
eos quota ls -m -n
```
```
quota=node uid=xxxx space=/eos/test/ usedbytes=0 usedlogicalbytes=0 usedfiles=2 maxbytes=1000000000000 maxlogicalbytes=500000000000 maxfiles=1000000 percentageusedbytes=0.00 statusbytes=ok statusfiles=ok
```