[Improvement] Add Structured Logging

[vincentkoc]

Description

Replace file-based logging with structured logging for better observability.

Current State

• File-based logging to /tmp/opik-mcp.log
• Simple text format
• Limited metadata

Target State

• Structured JSON logging
• Multiple log levels
• Rich metadata
• Log rotation
• Multiple outputs

Implementation Options

1. pino (recommended - fast, low overhead)
2. winston (feature-rich)

Tasks

• Choose logging library (pino recommended)
• Replace all logToFile() calls
• Configure log levels (error, warn, info, debug, trace)
• Add structured metadata to logs
• Configure log rotation
• Add multiple outputs (file, console, remote)
• Update configuration for log level control
• Update documentation

Example with Pino

import pino from 'pino';

const logger = pino({
  level: config.logLevel || 'info',
  transport: {
    target: 'pino-pretty',
    options: { colorize: true }
  }
});

logger.info({ tool: 'get-prompts', duration: 123 }, 'Tool executed');

Log Levels

• error: Errors that need attention
• warn: Warnings
• info: General information
• debug: Debug information
• trace: Detailed trace information

Benefits

• Easier log parsing
• Better debugging
• Integration with log aggregators
• Performance insights