[FEATURE]: Drop labels for metrics

[EvanLjp]

The iLogtail metrics labels is merged in fixed field called __labels__.

__labels__:cluster#$#sls-mall|endpoint#$#http-metrics|instance#$#192.168.32.71:10255|job#$#kubelet|le#$#0.256|namespace#$#kube-system|node#$#cn-beijing.192.168.32.71|service#$#kubelet|url#$#https://192.168.32.45:6443/api/v1/nodes/%7Bname%7D|verb#$#GET

we want to drop labels by processor.