[clangd] Add CSV export for trace metrics

Summary: Example: https://docs.google.com/spreadsheets/d/1VZKGetSUTTDe9p4ooIETmdcwUod1_aE3vgD0E9x7HhI/edit

Reviewers: kadircet

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, jfb, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79678