Add additional process metrics to the metrics semantic conventions

[evan-bradley]

What are you trying to achieve?

I would like to add the following metrics to the metrics semantic conventions:

• process.open_file_descriptors: Count of file descriptors a process has open
• process.voluntary_ctx_switches: Count of times a process was voluntarily context switched
• process.involuntary_ctx_switches: Count of times a process was involuntarily context switched

Additional context.

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

For the context switch metrics, I named them with the ctx abbreviation for "context" since that appears to be standard with vendors and in Linux, where it is abbreviated as ctxt. Additionally, things such as "input/output" are abbreviated to "io" in the process metrics semantic conventions. However, if it would be preferred, I think the full name of context would also be fine for the metric names.

I have opened a PR for this change here: https://github.com/open-telemetry/opentelemetry-specification/pull/2706