Disable GC profiling on Ruby 3 by default due to Ruby VM Ractor-related bug

[ivoanjo]

What does this PR do?:

This PR:

• Adds a toggle that can be used to enable/disable the GC profiling feature added in [PROF-6260] Enable tracking of time spent in GC #2336
• Enables the GC profiling feature by default on Ruby 2.x and disables the feature by default on Ruby 3.x
• Exposes a new DD_PROFILING_FORCE_ENABLE_GC / settings.profiling.advanced.force_enable_gc_profiling setting to allow customers that don't use Ractors to manually re-enable the GC profiling feature.

Motivation:

While validating that the new profiler correctly works with Ractors, I discovered that adding specs that invoked Ractors caused the previously-built GC profiling feature (and attached specs) to start failing.

I was able to track this down to an actual VM bug, which I've reported upstream as https://bugs.ruby-lang.org/issues/19112.

To avoid impacting any customers (specifically, they would potentially observe profiles with incomplete data), I decided to disable GC profiling for Ruby 3.x customers for now.

I am hoping that the Ruby core team can be convinced to fix this issue and backport it to existing Ruby 3.x stable releases, so that we can again re-enable this feature for these users.

As an alternative, if this issue sticks around for a long time and we really want this feature to be on by default for Ruby 3.x customers, there are probably are ways for us to automatically detect if Ractors are in use or not. I chose not to pursue such a mechanism for now.

Additional Notes:

(None)

How to test the change?:

The change includes test coverage. You can also observe this feature by profiling a Ruby application:

• On Ruby 2.x you will see garbage collection frames
• On Ruby 3.x you will not see garbage collection frames unless you force enable the feature.

[ivoanjo]

For context, the 2 to 3 change here is the number of positional arguments to the method. This is documented here.

Effectively there’s three ways to “wire up” a C function to be called as a Ruby method:

• if you pass in a number >= 0 then that’s the number of positional arguments to the method. E.g. def foo(a, b, c) => rb_define_method(..., foo_implementation, 3) and foo_implementation is expected to have a signature that receives self + 3 other arguments, e.g. VALUE foo_implementation(VALUE self, VALUE a, VALUE b, VALUE c).
• if you pass in -1 then Ruby will pass the arguments as a C array
• if you pas in -2 then Ruby will pass in the arguments as a Ruby array (e.g. it’s the C equivalent of def foo(*args))

I moved it from 2 to 3 since I was passing the new gc_profiling_enabled value