Add YJIT metrics to `Process`

[erickguan]

Hey!

I want to add a few metrics about YJIT in the Process instrumentation to understand the YJIT performance.

These metrics depend on the RUBYOPT=--yjit-stats flag and Ruby::YJIT.runtime_stats API.
When the Ruby process starts with --yjit-stats or RUBYOPT=--yjit-stats, the Ruby::YJIT.runtime_stats produces performance stats. Otherwise, Ruby::YJIT.runtime_stats returns nil. Naturally, I would propose to add:

1. A new function collect_yjit_metrics in the Process#collect.
2. collect_yjit_metrics collects:
   • ratio_in_yjit: the percentage of total YARV instructions executed by YJIT as opposed to the CRuby interpreter
   • code_region_size
   • yjit_alloc_size
   • code_gc_count
   • compiled_iseq_count
   • yjit_insns_count
   • side_exit_count
   • total_exit_count
   • avg_len_in_yjit

Reference: Ruby YJIT Docuementation