Implement StatsCollector for basic metrics aggregation

[marr97]

Description

Thread-safe component that collects per-request results and provides aggregated metrics snapshot.
Will be integrated with Engine later.

Tasks

• StatsCollector class with
  • Thread-safe counters using LongAdder: totalRequests, successCount, errorCount
  • Latency tracking: min, max, totalLatency (for average)
  • record(int statusCode, long latencyMs) method
  • getSnapshot() method returning current aggregated metrics
  • reset() method for clearing state between tests
• StatsSnapshot record/class with fields: totalRequests, successCount, errorCount, avgLatencyMs, minLatencyMs, maxLatencyMs
• Unit tests, more info below

Unit tests

record

• throws on negative status code
• throws on status code below 100
• throws on status code above 599
• throws on negative latency
• boundary: 200 is success, 199 is error, 299 is success, 300 is error
• zero latency is valid

getSnapshot

• before any records returns all zeros

record + getSnapshot

• single success record counted correctly
• single error record counted correctly
• multiple records — all counters correct
• average latency calculated correctly
• min latency tracked correctly
• max latency tracked correctly

record + getSnapshot + reset

• records after reset are independent of previous state
• 10 cycles with random count of records (1-10), random statusCode (100-599), random LatencyMs (0-999)
• double reset is safe
• snapshot taken before reset is not affected by reset

Thread safety

• concurrent writes from multiple threads produce correct total count
• 10 cycles, 5 threads, random records per thread (10-109), random statusCode (100-599), random latencyMs (0-999)