Refactor metadata.go for improved maintainability

[harp-intel]

Summary

Split metadata collection logic into architecture-specific files to improve code organization and make it easier for new contributors to navigate the codebase.

Changes

File reorganization:

• metadata.go - Common types, interfaces, script definitions, and shared helper functions
• metadata_x86.go - x86_64 (Intel/AMD) metadata collector
• metadata_aarch.go - ARM/aarch64 metadata collector

Code quality improvements:

• Extract script name constants (16 total) to prevent silent failures from typos when accessing script outputs by name
• Move script definitions to package-level baseMetadataScripts var for clearer separation from parameterization logic
• Simplify getMetadataScripts() to copy and configure base definitions

Benefits

• New developers can quickly find architecture-specific code
• Script name typos now cause compile errors instead of silent runtime failures
• Reduced file size: metadata.go went from ~1043 lines to ~513 lines
• Clearer separation of concerns

Test plan

• make test passes
• go build ./... succeeds

[harp-intel]

Additional improvements in this update

• Add scriptPerfStatFixedPrefix constant and use it in getSupportsFixedEvent()
• Add defensive checks for cpuInfo map access in x86 collector (explicit key existence checks with clear error messages)
• Fix ARM CPUSocketMap to support multi-socket systems by reading from sysfs instead of assuming single socket
• Remove redundant warning logs in getARMSlots() (errors already convey the information)

Additional benefits

• Better error messages when /proc/cpuinfo fields are missing
• Correct CPU-to-socket mapping on multi-socket ARM systems