perf(profiling): allocate 512 capacity initially for alloc map

[gyuheon0h]

Description

flat_hash_map rehashes at 87.5% load (https://abseil.io/docs/cpp/guides/container#hash-tables) doubling capacity each time. Without this, growing from 0 to 1024 entries triggers ~8 rehashes:
capacity: 4 -> 8 -> 16 -> 32 -> 64 -> 128 -> 256 -> 512 -> 1024
rehash at: 4 7 14 28 56 112 224 448

p99 live heap count is ~350

Each rehash at capacity N copies all N existing entries.

This only minorly improves startup, but this is a cheap non-negative change

Testing

Risks

Additional Notes

[gyuheon0h]

• perf(profiling): allocate 512 capacity initially for alloc map #18119 👈 (View in Graphite)
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[cit-pr-commenter-54b7da]

Codeowners resolved as

ddtrace/profiling/collector/_memalloc_heap.cpp                          @DataDog/profiling-python

[datadog-official]

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 2242e8c | Docs | Datadog PR Page | Give us feedback!}

[KowalskiThomas]

I have nothing against this but FWIW I've never seen rehashes show up in profiles so it's probably not going to change much.