Refactor interface for registering custom label providers

[patnebe]

Context

• The interface for registering custom providers assumes all metadata providers will accept a pid arg. This is not true, especially for providers that return system-level metadata that might be company/deployment/runtime environment specific and not in any way related to the task being profiled.
• To work around this today, one option is for users have to modify metadata_provider.rs in their fork to add/invoke logic for custom system metadata providers. This is not ideal.
• A separate interface for registering system-level metadata providers eliminates the need for such code changes.

Changes

• Rename ThreadSafeMetadataProvider to ThreadSafeTaskMetadataProvider
• Introduce a new SystemMetadataProvider trait that has a get_metadataaccepts no arguments and returns a vector of labels.
• Add a new method -- register_system_metadata_providers() -- to the GlobalMetadataProvider struct.

Test Plan

• Manual testing + Existing Unit tests.

[javierhonduco]

Makes sense, thanks for the PR!