v0.8.3

Fixes

1. Per-DataStorage metrics page memory leak. Each DataStorage registered a metrics page in the global C++ metrics storage and only detached it on destruction, but detached pages are physically reclaimed only during C++ metrics collection — which was disabled by default in v0.8.2. With the collector off, every created-and-destroyed DataStorage leaked its metrics page (observed as steady heap growth on stage). DataStorage now owns its metrics object directly instead of registering it globally, so no page can leak. As a consequence the per-DataStorage prompp_data_storage_* metrics are no longer exposed to the C++ metrics collector.
2. C++ metrics collector re-enabled by default. The v0.8.2 use-after-free mitigation disabled the C++ metrics collector by default (PROMPP_FEATURES=enable_cpp_metrics) because it iterated metrics-page memory that could be freed concurrently by remove_unused_pages. Now that DataStorage no longer registers pages in the global metrics storage, nothing populates it in production, so that concurrent-free path is gone and the collector is safe to run unconditionally again. The enable_cpp_metrics feature flag is removed.