Logflare / logflare / 0d5de0329ba4def66cb2302f075ab767c7701b02

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perf(cleaner): flatten metadata without per-level intermediate lists (#3456)

* test(cleaner): cover empty-leaf, nested-list, and mixed-list flatten cases

The existing flatten/1 tests do not assert behavior around empty
containers at leaves (the docstring promises they are preserved as
values), nested lists, or lists with mixed element types. Add three
focused cases so any future rewrite has a behavioral fence.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* perf(cleaner): build flatten map without intermediate nested lists

Rewrites MetadataCleaner.flatten/1 as an accumulator-passing tail recursion
that builds a single flat list of {key, value} pairs and finalizes with
:maps.from_list/1, replacing the Enum.flat_map / Enum.with_index pipeline
that allocated nested per-level lists plus an index-paired tuple list before
concatenation. Reverses the accumulator before :maps.from_list/1 so
duplicate-key collisions resolve identically to the previous
Map.new-over-traversal-order behavior.

Adds a focused profiling script to attribute flatten cost directly.

Focused flatten bench (Apple M5, median):
  otel trace                       800 → 440 reductions   (-45%)
  edge log    38.4 KB /  2,390  → 31.3 KB /  1,580        (-18% mem, -34% reductions)
  list heavy 202.6 KB / 13,930  → 156.5 KB /  9,430       (-23% mem, -32% reductions)

End-to-end LogEvent.make/2 bench (Apple M5, median):
  otel trace  80.6 μs /  31.29 K → 78.3 μs /  30.91 K  (-1.2% reductions)
  edge log   304.1 μs / 137.36 K → 298.0 μs / 136.67 K (-0.5% reductions)

Reduction-count deltas are deterministic; wall-time / memory differences
in LogEvent.make/2 sit within run-to-run noise (±8-12% deviation) on this
hardware. Flatten is a small share of the full make pipeline — proportional
wins land in the focused bench.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* refactor(cleaner): tighten flatten internals from review feedback

- Pattern matc... (continued)

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