add documentation for the new metrics

[matsumana]

I want to update README.md about #85 and #88 after #88 will be merged.
so I have a question.

Would you tell me the meaning of cardinality values in README.md?

[zwopir]

sure. the cardinality of a metric is the number of metrics you get if you count all possible label-metric combinations. So if you have a metric foo with a label label_foo and that label can be value1, value2 and value3 the cardinality of the metric would be 3. If you have multiple labels you simply multiply the numbers of possible label_values of each label. If you are unsure, just add the metric to the README.md and leave the cardinality blank, I can then add the value later on

[matsumana]

Thank you. I will send PR.