Propose new conventions

I would like to add 3 new CPython metrics regarding its garbage collector:

• cpython.gc.collections
• metric.cpython.gc.collected
• metric.cpython.gc.uncollectable

And then a an attribute cpython.gc.generation to track for what gc generation (gen0, gen1, gen2) this metrics are.

Data will come from the gc.get_stats API: https://docs.python.org/3/library/gc.html#gc.get_stats