instrumentation-runtime-node performance

[trentm]

Anecdata suggests this instr can have an impact on performance (in one piece of anecdata it was about startup time). Let's investigate this a bit. Separately, add mention of this to a performance-tuning and/or troubleshooting doc.

[trentm]

FWIW PerformanceObserver docs (https://nodejs.org/api/perf_hooks.html#new-performanceobservercallback) warns about potential overhead from running this continuously:

Because PerformanceObserver instances introduce their own additional performance overhead, instances should not be left subscribed to notifications indefinitely. Users should disconnect observers as soon as they are no longer needed.

and the gcCollector.ts in instr-runtime-node is doing just that. I'm not sure about startup vs. steady-state overhead though.