add an efficient labels serializer

[ftelnov]

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What has been done? Why? What problem is being solved?

I found out that observe method works slow. Further investigations showed that make_key function sorts keys every run time. make_key is being called often during, for instance, hist:observe. To optimize this case and both keep the default behavior, I propose two things:

1. Allow user to override labels_pairs serialization scheme with __metrics_make_key meta-method. If it is supplied, it would be used, otherwise the default sorting scheme would be used;
2. Provide precaching labels serializer. It would order the labels properly on the client side, and only concat pairs on the metrics side. The complexity is linear then.

I didn't forget about

• [x] Tests
• [x] Changelog
• [ ] Documentation (README and rst) (don't think I should alter it for such a small api?)
• [ ] Rockspec and rpm spec (I don't really know what to add here)

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