Add performance metrics and comparison between API client implementations

[hossain-khan]

Problem

No visibility into performance differences between Retrofit and GH CLI implementations:

• Which is faster for various operations?
• Network usage comparison
• API rate limit consumption patterns
• Memory usage differences

Proposed Solution

Add performance tracking and comparison metrics:

1. Extend CacheStatsService

Add implementation-specific metrics:

data class ApiClientPerformanceStats(
    val clientType: ApiClientType,
    val totalRequests: Int,
    val averageResponseTime: Duration,
    val cacheHitRate: Double,
    val networkBytesTransferred: Long,
    val rateLimitRemaining: Int
)

2. Add Instrumentation

Wrap API calls with timing:

suspend fun <T> measureRequest(operation: suspend () -> T): Pair<T, Duration> {
    val start = System.nanoTime()
    val result = operation()
    val duration = Duration.ofNanos(System.nanoTime() - start)
    return result to duration
}

3. Generate Comparison Report

Create a summary showing:

• Average request time per implementation
• Cache effectiveness
• Network efficiency
• Rate limit usage

Example output:

API Client Performance Comparison
==================================
Retrofit Implementation:
  - Avg Response Time: 145ms
  - Cache Hit Rate: 73%
  - Network Requests: 27
  
GH CLI Implementation:
  - Avg Response Time: 285ms
  - Cache Hit Rate: 0%
  - Network Requests: 100

Benefits

• Data-driven decision making for implementation choice
• Identify performance bottlenecks
• Guide optimization efforts
• Document tradeoffs for users

Priority

Low - Nice to have but not critical for functionality

Related Files

• src/main/kotlin/dev/hossain/githubstats/client/GitHubApiClient.kt
• src/main/kotlin/dev/hossain/githubstats/cache/CacheStatsService.kt