Add metric for pg_dsm_registry_allocations

[nadashaban11]

Description

Added metric for dsm_registry_allocations to report dynamic shared memory allocations using the new pg_dsm_registry_allocations view provided in PostgreSQL 19.

Fixes #1400

• SQL query to fetch the allocations retrieved from pg_dsm_registry_allocations view
• Used coalesce(size, 0) to handle edge cases, as I read that partially-initialized entries will report a NULL size
• Applied tag naming (tag_name, tag_type)
• Registered the new metric in the presets (exhaustive, full, debug, exhaustive_no_python)
• I set is_instance_level: true based on my understanding that DSM is a cluster-wide resource, but please let me know if this needs correction

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Checklist

• Code compiles and existing tests pass locally.
• New or updated tests are included where applicable.
• Documentation is updated where applicable.