Standardize metric labels and units

[bialesdaniel]

Addresses issue #337. I'm not sure if these are the labels that we want. I just picked one type of label and went with it. I actually think the snake case might be better. I am not sure if this breaks anything with Matlab. I also wasn't really sure how to test this. Also, the documentation is out of date https://github.com/oltpbenchmark/oltpbench/wiki#output

[apavlo]

@ZiqiDong97 @AlanDZQ We want to merge this after we get your PR in

[bialesdaniel]

I just checked whether this change would break any NoisePage scripts. It looks like it won't break anything.

If it were to break anything the problem would be in: https://github.com/cmu-db/noisepage/blob/999dea0b3b790f7ff56eec7e133ad514e3958e20/script/testing/reporting/utils.py#L4 or https://github.com/cmu-db/noisepage/blob/master/script/testing/reporting/constants.py because of calls from https://github.com/cmu-db/noisepage/blob/master/script/testing/reporting/parsers/oltpbench/summary_parser.py#L78 and https://github.com/cmu-db/noisepage/blob/master/script/testing/reporting/parsers/oltpbench/res_parser.py