data metrics v2: add rate metrics

[majewsky]

I adjusted the testcases to rename xOtherRate to a more plausible name to make the testcase more readable.

[github-actions]

Merging this branch will increase overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/sapcc/limes/internal/api	78.88% (ø)
github.com/sapcc/limes/internal/api/api_v2	74.81% (ø)
github.com/sapcc/limes/internal/collector	83.37% (+0.36%)	👍
github.com/sapcc/limes/internal/core	78.23% (+0.40%)	👍
github.com/sapcc/limes/internal/db	73.75% (ø)

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Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/sapcc/limes/internal/api/api_v2/info.go	92.86% (ø)	686	637	49
github.com/sapcc/limes/internal/api/rate_updater.go	47.19% (ø)	623	294	329
github.com/sapcc/limes/internal/collector/metrics.go	87.86% (+0.39%)	3633 (+448)	3192 (+406)	441 (+42)	👍
github.com/sapcc/limes/internal/core/config.go	100.00% (+4.82%)	581	581 (+28)	0 (-28)	👍
github.com/sapcc/limes/internal/db/models.go	100.00% (ø)	78	78	0

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.

Changed unit test files

• github.com/sapcc/limes/internal/collector/quota_overrides_test.go
• github.com/sapcc/limes/internal/collector/scrape_test.go

[VoigtS]

The logic looks good to me, I only have a small remark on the ServiceRateLimitConfiguration method comments.