feat: replace prometheus/log package with log

[hanyuancheung]

Background:
There are some old issues, between package github.com/sirupsen/logrus and github.com/Sirupsen/logrus.
When I import the clickhouse_exporter package into my project, I met the problem with: case-insensitive import collision.

I've searched several infos, like golang/dep#1010
clickhouse_exporter seems import sirupsen package because of the past-out prometheus/log package, which is already renamed and archived. https://github.com/prometheus-junkyard/log

So, I think clickhouse_exporter should remove prometheus/log package instead of using log package.

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[hanyuancheung]

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