feat: add API event recorder interface for billing

[zhangzhenghao]

Summary

Add common/event package for recording API call events, designed for future billing/metering functionality.

Changes

• APIEvent struct: Captures request metadata, response info, and billing quantities (users, items, feedbacks, recommendations)
• Recorder interface: Abstract interface for event storage backends (database, message queue, external service)
• NopRecorder: No-op implementation when recording is disabled
• BufferRecorder: In-memory buffer for testing and debugging
• Global recorder: SetRecorder/GetRecorder/Record functions for easy integration

Usage

// Set recorder
event.SetRecorder(event.NewBufferRecorder())

// Record event
event.Record(ctx, event.APIEvent{
    RequestID:   requestId,
    APIKey:      apikey,
    Method:      "GET",
    Path:        "/api/recommend/user123",
    StatusCode:  200,
    ResponseTime: 45,
    RecommendCount: 10,
})

Future Extensions

• DatabaseRecorder: Persist events to MySQL/PostgreSQL/MongoDB
• KafkaRecorder: Send events to message queue
• BillingServiceRecorder: Push to external billing API

Testing

• Build passes: go build ./common/event/
• No additional dependencies added

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.95%. Comparing base (d54a2ea) to head (ec64ce7).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1266      +/-   ##
==========================================
+ Coverage   72.89%   72.95%   +0.05%     
==========================================
  Files          91       92       +1     
  Lines       16743    16764      +21     
==========================================
+ Hits        12205    12230      +25     
+ Misses       3293     3288       -5     
- Partials     1245     1246       +1     

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[Copilot]

Pull request overview

Introduces a new common/event package intended to record API and storage usage events for future billing/metering, and wires it into the REST access filter and master dataset-loading path.

Changes:

• Added common/event with APIEvent/StorageEvent types, a Recorder interface, and a global recorder setter/getter (defaulting to NopRecorder).
• Hooked API event recording into server.RestServer.LogFilter.
• Hooked storage event recording into master.(*Master).loadDataset, plus added basic unit tests for recorder swapping.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 6 comments.

File	Description
server/rest.go	Records API call metadata via the global event recorder from the REST logging filter.
master/tasks.go	Records storage usage counts after dataset load via the global event recorder.
common/event/event.go	New event types + recorder interface + global recorder wiring (defaults to no-op).
common/event/event_test.go	Adds a mock-based test verifying SetEventRecorder/EventRecorder behavior.

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