[PROF-8667] Heap Profiling - Part 4 - Labels

[AlexJF]

What does this PR do?

This PR follows #3328 by adding allocation class and gc gen age labels to the heap samples included in the Ruby profiles.

How does it work?

For allocation class, we simply hold on to the resolved class that was already being computed during the normal allocation sampling path.

For gc gen age, we record the GC Generation/Epoch (i.e. GC.count) when we saw the allocation happening. Then, when writing the heap samples in a new profile, we calculate the difference between the allocation generation and the current generation and that gives us the age of the recorded object in terms of "how many GCs have run since we saw this object being allocated".

Motivation:

• Understand the type of objects that are being kept alive in our heap.
• Understand the age dynamics of the objects being kept alive in heap (are they all "new" and likely

Additional Notes:

How to test the change?
Heap samples included in profiles when an app is executed with:

DD_PROFILING_EXPERIMENTAL_ALLOCATION_ENABLEd=true DD_PROFILING_EXPERIMENTAL_HEAP_ENABLED=true ddtracerb exec ...

should now include the above labels. This can be trivially tested by downloading these profiles and looking at them with some pprof parsing tool like https://github.com/felixge/pprofutils

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[ivoanjo]

Left some notes, but this looks sane! 👍

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (28dc187) 98.23% compared to head (1a2d02d) 98.23%.
Report is 3 commits behind head on master.

Additional details and impacted files

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##           master    #3329      +/-   ##
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- Coverage   98.23%   98.23%   -0.01%     
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  Files        1253     1253              
  Lines       73039    73048       +9     
  Branches     3431     3428       -3     
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+ Hits        71752    71760       +8     
- Misses       1287     1288       +1     

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[ivoanjo]

LGTM 👍 Left a few final comments/suggestions