Fix use-after-free of DataStorage metrics page address label

[vporoshok]

Summary

Fixes intermittent invalid (non-UTF-8) values for the address label of prompp_data_storage_* metrics, which made prompp's self-scrape fail to parse its own metrics (invalid utf8 in label_value ... prompp_data_storage_asc_int_count{address=...}).

Two related root causes:

1. Dangling label view (use-after-free). The address label value was a non-owning Go::String view into a std::string stored inside DataStorage. On DataStorage destruction the metrics page was only detached (deferred removal), but the backing string was freed immediately in the destructor. A concurrent scrape that already advanced onto the page would read freed/reused memory — hence the rare garbage bytes. Ownership of the address string is moved into the Metrics page, so its lifetime matches the page and it is reclaimed only by remove_unused_pages().

2. Unserialized scrapes. client_golang does not serialize Gather/Collect, so two concurrent scrapes could race: prompp_metrics_iterator_ctor calls remove_unused_pages() (which physically deletes detached pages), so one scrape could delete a page another scrape is still iterating. A mutex now serializes the whole CppMetrics iteration, guaranteeing the single-reader invariant the detach/remove design relies on.

Changes

• pp/series_data/metrics.h — Metrics page owns address_label_ (moved in); metrics built from a label_set() helper viewing the page-owned string.
• pp/series_data/data_storage.h — drop the address_label_ member from DataStorage; pass the address string to the page at construction.
• pp/go/cppbridge/metrics.go — package-level sync.Mutex held for the full CppMetrics iteration.
• pp/series_data/tests/metrics_tests.cpp — regression test asserting the address label stays valid after the owning DataStorage is destroyed.

Test plan

• make build-entrypoint (ARM devcontainer)
• C++ //:metrics_test, //:series_data_test pass
• New lifetime regression test passes, also under --asan=true
• Go ./pp/go/cppbridge metrics tests pass
• gofmt clean

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