fix: use keyed fields in SamplePair struct literals in api_test.go

[immanuwell]

go vet ./... fails on 15 unkeyed model.SamplePair struct literals in TestSamplesJSONSerialization. The bench test in the same package already uses keyed syntax, so this just brings api_test.go in line with it.

Reproduce:

go vet ./api/prometheus/v1/...
# api/prometheus/v1 [api/prometheus/v1.test]
api/prometheus/v1/api_test.go:1622:14: github.com/prometheus/common/model.SamplePair struct literal uses unkeyed fields
...

Fix: add Timestamp: and Value: field names to all 15 literals. No behavior change, tests still pass.

@bwplotka @beorn7

[kakkoyun]

LGTM. However, mac os tests are failing. We need to check.

[Copilot]

Pull request overview

This PR updates TestSamplesJSONSerialization to use keyed field names (Timestamp: and Value:) in model.SamplePair struct literals, aligning the test with go vet expectations and existing usage patterns in the package.

Changes:

• Replaced 15 unkeyed model.SamplePair{...} literals with keyed equivalents to satisfy go vet ./....
• Kept test semantics unchanged (same timestamp/value inputs and expected JSON output).

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