DO NOT MERGE: Telemetry

[gmalette]

I had started working on telemetry code to gain insight into how esbuild-dev is actually used and what kind of usage characteristics we can expect. I was very interested in knowing how many files are compiled, and how many are loaded, and the ratio of these two numbers.

I moved on to other things in the meantime, and I had issues where the SyncWorker span seemed to be delayed by a large margin.

I figure there's value in keeping this around, we'll likely want to have it at some point.