Add IORuntimeMetrics and CpuStarvationMetrics

[iRevive]

The first step towards cross-platform IORuntimeMetrics. This PR also introduces a cross-platform CpuStarvationMetrics.

Motivation

It would be nice to have an option to access runtime metrics without relying on MBeans. For example, you can use Scala OpenTelemetry SDK to collect starvation metrics on all platforms (JVM, JS, Native).

Usage

IO.delay(runtime.metrics.cpuStarvation.maxDrift())
  .flatMap(drift => IO.println(s"current drift: $drift"))
  .delayBy(2.seconds)
  .foreverM

Next steps

Add WSTP compute and local queue metrics. The draft - #3317.

[iRevive]

Long could be more suitable and more memory efficient.

[djspiewak]

What do you think about moving this stuff to cats.effect.unsafe.metrics? That feels like a more appropriate place, since we really don't want to wrap the impure interfaces in IO, but at the same time it's absolutely not a "safe" set of APIs in the sense we normally use the word.

[iRevive]

Good point. I moved classes to the cats.effect.unsafe.metrics package.