[proemtheus-remote-write] fix label sorting

[tals]

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Description

This is required by the spec "MUST have label names sorted in lexicographical order."

This library would inject the __name__ label at the end. This broke the label ordering, leading Thanos to reject the write with a "409 Conflict" error.

Type of change

Please delete options that are not relevant.

Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Unit test, and tested against a Thanos instance.

Does This PR Require a Core Repo Change?

• [ ] Yes. - Link to PR:
• [x] No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• [x] Followed the style guidelines of this project
• [x] Changelogs have been updated
• [x] Unit tests have been added
• [x] Documentation has been updated

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• :white_check_mark: login: tals / name: Tal Shiri (a69529034051957a027d5731c2bafa45f2230936)

[ranvit]

+1 thanks for making this PR @tals I had implemented the same fix on a fork and its been working well for me.

This is certainly a needed feature because Thanos chose to define and assert the missing remote-write spec - see here, one of the assertions being lexicographically sorted label names

cc'ing @lzchen