Add request logging with structured JSON output #17

Summary

Add structured JSON logging for all API requests to improve debugging and monitoring in production.

Problem

Currently, the Gateway only logs basic startup messages. There's no visibility into:

• Incoming requests (method, path, IP)
• Response status codes and latency
• Payment verification outcomes
• Error details

Proposed Solution

Add middleware that logs every request in structured JSON format:

{
  "timestamp": "2024-01-01T12:00:00Z",
  "method": "POST",
  "path": "/api/ai/summarize",
  "status": 200,
  "latency_ms": 45,
  "client_ip": "192.168.1.1",
  "payment_verified": true,
  "user_wallet": "0x123..."
}

Architecture

flowchart LR
    REQ[Incoming Request] --> LOG[Logging Middleware]
    LOG --> HANDLER[Route Handler]
    HANDLER --> LOG
    LOG --> RES[Response + Log Entry]
    
    LOG --> STDOUT[(stdout / JSON)]

sequenceDiagram
    participant C as Client
    participant L as Logger Middleware
    participant H as Handler
    
    C->>L: Request
    L->>L: Record start time
    L->>H: Forward request
    H-->>L: Response
    L->>L: Calculate latency
    L->>L: Write JSON log
    L-->>C: Response

Acceptance Criteria

• Add logging middleware to Gateway
• Log request method, path, status, and latency
• Log payment verification status (402/403/200)
• Include client IP address (handle X-Forwarded-For)
• Use JSON format for easy parsing by log aggregators
• Add log level configuration via environment variable
• Don't log sensitive data (signatures, API keys)
• Include unit tests for the logging middleware

Technical Notes

• Gin has built-in gin.Logger() middleware as reference
• Consider using zerolog or zap for structured logging
• Log to stdout for container compatibility

Testing

# Run gateway and observe logs
go run main.go

# Make request and verify log output
curl -X POST http://localhost:3000/api/ai/summarize \
  -H "Content-Type: application/json" \
  -d '{"text": "test"}'

# Expected log entry in stdout