prometheus / client_ruby / 533

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Pull #164

travis-ci

Fix bug where different label orders lead to different results

In the DirectFileStore, in order to store the labels hash on disk, we
turn it into a "querystring". The way this is done, we'll end up with
different strings if the same hash is passed in with its keys in
different order.

Since this string is used to index into the file where we store the data,
this will lead to two different values being stored for the same hash.
This is fine in cases where the aggregation is :SUM, because they end up
getting summed when the Client is summarizing. But in :ALL aggregation,
for example, you will end up with one value or the other, randomly.

The test in this commit reproduces this problem.

This way of serializing the labels is a bit slower (see PR for details),
but it's not a huge impact in the big scheme of things, and it leads to
the correct result.

Signed-off-by: Daniel Magliola <danielmagliola@gocardless.com>

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