perf(transform): avoid redundant allocations when grouping events

[0xbrayo]

Grouping events currently allocates an unused map for each new group, clones the first event's JSON, and performs redundant hash lookups. Use HashMap::entry and move that event into the group, preserving the first timestamp and payload, clearing its ID, and accumulating later durations.

Part of #671 (grouping allocations).

Validation:

• cargo test -p aw-transform: 50 tests passed.
• Added a regression for multiple keys, missing keys, nested payloads, first-event metadata, and ID clearing.
• Added Criterion cases for 50,000 events with 100 and 50,000 distinct groups, with input setup outside the timed operation. Local optimized timings were approximately 9.4 ms and 8.9 ms respectively. An earlier same-process prototype comparison measured 22.6 ms → 10.3 ms for the all-distinct case; these are synthetic measurements, not application-wide speedups.

This retains the existing grouping-key encoding and unordered output.

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge, with no outstanding correctness, security, or repository-rule violations.

Summary

• Uses HashMap::entry to avoid redundant lookups.
• Moves the first event into each group instead of cloning its payload.
• Clears the retained event ID and accumulates subsequent durations.
• Adds regression coverage and Criterion benchmarks for low- and high-cardinality grouping.

_{Reviews (2) · Last reviewed commit: "perf(transform): avoid redundant allocat..."}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.74%. Comparing base (656f3c9) to head (8b7d519).
⚠️ Report is 103 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #673      +/-   ##
==========================================
+ Coverage   70.81%   79.74%   +8.93%     
==========================================
  Files          51       67      +16     
  Lines        2916     5851    +2935     
==========================================
+ Hits         2065     4666    +2601     
- Misses        851     1185     +334     

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[0xbrayo]

@greptile review