Add some GC stats to Py_STATS

[markshannon]

This partly duplicates the stats from the GC module, but serves a different purpose, as it allows us to gather stats that would be too expensive to gather in a release build.

The stats:
https://github.com/faster-cpython/benchmarking/blob/main/results/bm-20230730-3.13.0a0-8bc0926/bm-20230730-azure-x86_64-faster%252dcpython-gc_stats-3.13.0a0-8bc0926-pystats.md

[pablogsal]

I have already a PR open for this:

#100958

I would prefer to complete that one if possible

[markshannon]

@pablogsal
If you want more stats could you use the Py_STATS mechanism?
That way the GC stats can be gathered along with all the other stats.

If you need additional timings, then adding an -X mechanism makes sense.
In that case I don't think that we want stats as the additional overhead will mess up the timings.

[markshannon]

@pablogsal
I'd like to get this merged, as it just adds a few stats using the Py_STATS mechanism, whereas #100958 seems much broader.

There is some overlap between this PR and #100958, but #100958 has been open since January, and I'm worried that we will have no stats at all for months.
I'll be happy to review #100958 once it is mergeable.

As an aside:
Treating objects visited as a proxy for time taken, both your and my stats seem to show similar results.
Gen 2 is the most efficient, followed by gen 1 then gen 3.

[pablogsal]

Ok let's merge this PR and let's refine the other one 👍

[iritkatriel]

LGTM