Add middleware to record YJIT code_region_size

[errm]

• We are using the rack.after_reply hook for this to not impact response times - https://github.blog/2022-04-11-performance-at-github-deferring-stats-with-rack-after_reply/
• Just a simple implimentation now, so we can optimise --yjit-exec-mem-size on applications, but this should be simple to extend to record the other metrics exposed by runtime_stats if they are needed.