[Canary] processor_labels: plugging issue of missing static labels on concatenation

[cosmo0920]

This PR is depending on fluent/cmetrics#302.

Also, added runtime tests and integration tests.

The root cause of this issue is:

Confirmed: the labels processor treats cmt_cat() as a full context clone, but it does not preserve context-level static labels. The local binary reproduces your result.

The failure path is:

1. original="value" is stored as a per-series label.
2. The first processor’s upsert sees that first is absent from the dynamic labels and adds it to cmt->static_labels. See upsert_labels().
3. The second processor creates an empty context and calls cmt_cat(out_cmt, metrics_context). See cb_process_metrics() line 1858.
4. `cmt_cat()` line 1093 copies metric families, including their per-series labels, but never copies src->static_labels. Thus original survives and first disappears before the second processor applies its operations.
5. The second processor adds second="two" to its new context.

Observed results:

Configuration	Exported labels
One processor: upsert first	original="value", first="one"
Two processors: upsert first, then upsert second	original="value", second="two"
One processor: upsert first and insert second	All three labels
Two processors: upsert first, then update original	Only original="changed"

The last case establishes that this affects any subsequent labels processor, even one that only updates an unrelated label.

The focused fix would be to deep-copy the incoming static labels into the new context before applying label operations, with allocation-failure cleanup. Changing cmt_cat() globally needs more consideration: it also merges contexts, and adding source labels globally could apply them to unrelated destination metrics.

Existing coverage misses this because the upsert test configures multiple operations on a single processor.

Verification:

• ctest --test-dir build -R processor_labels --output-on-failure — passed.
• MEMCHECK=1 python3 /tmp/repro_labels_chain.py — reproduced the same results under Valgrind. Memory check failed in the two chained cases, reporting DNS lookup and input-coroutine leaks at shutdown; those stacks do not establish a connection to the label-copy defect.

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  Testing
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  • Backport to latest stable release.

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