[9.4](backport #50019) perf: remove unnecessary allocations in hot-path processors

[mergify]

Remove redundant Clone() calls and deferred allocations in four hot-path processors. No behavioral changes — events produced are byte-identical to main.

• add_docker_metadata: Remove Clone() of locally-built map that has no other references. Also fix deprecated cgroup.NewReader → cgroup.NewReaderOptions.
• add_kubernetes_metadata: Reduce 3 full metadata Clones to 1 full + 1 container sub-clone. The removed clone was Clone().GetValue() where GetValue is read-only.
• timestamp: Defer parseError allocation to failure path. Success path is now allocation-free.
• publisher/processing: Remove dead event.Meta.Clone() whose return value was discarded.

Per-processor benchmarks:

Processor	Δ ns/op	Δ B/op	Δ allocs/op
add_docker_metadata	-30%	-35%	-26%
add_kubernetes_metadata	-52%	-55%	-55%
timestamp (single layout)	-11%	-14%	-17%

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works.
• I have added an entry in ./changelog/fragments using the changelog tool.

Disruptive User Impact

None. No behavioral changes. All event output is identical to main.

How to test this PR locally

go test -bench=BenchmarkAddDockerMetadata -benchmem ./libbeat/processors/add_docker_metadata/
go test -bench=BenchmarkKubernetesAnnotatorRun -benchmem ./libbeat/processors/add_kubernetes_metadata/
go test -bench=BenchmarkTimestamp -benchmem ./libbeat/processors/timestamp/

Supersedes #49761.

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This is an automatic backport of pull request #50019 done by Mergify.

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