[clangd] Metric tracking through Tracer

Summary: Introduces an endpoint to Tracer for tracking metrics on
internal events.

Reviewers: sammccall

Subscribers: ilya-biryukov, javed.absar, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78429