Update the vendoring of bmizerany/perks/quantile.

This is still from my own fork as there are still critical PRs that
bmizerany hasn't responded to yet.

Change-Id: Ib46e68ec9ecbae9423b1bfe1311569426dfc7ba0
Make extraction honor ms-precision timestamps.

The test touched by this commit exposed the bug already.

Change-Id: I303131eab841ae0ea9b1a727230e68754a4cd8fe
Add a few trailing commas to tests.

That is to prove that trailing commas work for the processor.

Change-Id: Ibecac811d47958387c9633df69bafbe284cf0b8f
Merge pull request #35 from prometheus/u-c-l/maintenance

Just a couple of minor things maintenance.
Add COWMetric and do some code cleanups.