Add additional process metrics to the metrics semantic conventions

[evan-bradley]

Changes

Fixes #2708

Add the following process metrics to the semantic conventions:

• process.voluntary_context_switches
• process.involuntary_context_switches
• process.open_file_descriptors

Related issues https://github.com/open-telemetry/opentelemetry-collector-contrib/issues/12482

This will need to be rebased on https://github.com/open-telemetry/opentelemetry-specification/pull/2705 before it can be merged.