Feature/target rebalance

[GoneLikeAir]

Hi, recently I have finish a feature: target rebalance. It can make the load of prometheus more balance.

Why:

1. The access frequency of deferent job is not the same;
2. The series of targets in one group are close;
3. If most of the targets in one group assign to one shard, the load of prometheus may not balance.

How:

1. Group the target by job;
2. Check the distribution status of the targets, judge whether it is balanced(check the diff between Max and Min shard);
3. If not, move target from max shard to min shard.

This is the effect of this feature:
Before:

After: