Performance: Add metrics and monitoring instrumentation

[sfloess]

Overview

Add application metrics to track performance, cache hit rates, and usage patterns for production monitoring.

Current State

• No metrics collection
• No monitoring instrumentation
• Performance characteristics unknown in production
• Cache effectiveness unmeasured

Proposed Metrics

1. Cache Metrics

public class CacheMetrics {
    private final AtomicLong hits = new AtomicLong(0);
    private final AtomicLong misses = new AtomicLong(0);
    private final AtomicLong evictions = new AtomicLong(0);
    
    public double getHitRate() {
        long total = hits.get() + misses.get();
        return total == 0 ? 0 : (double) hits.get() / total;
    }
}

2. HTTP Metrics

• Request count by operation (list/delete/stats)
• Response times (p50, p95, p99)
• Retry count
• Error rate

3. Repository Metrics

• Components listed
• Components deleted
• Average component size
• Largest operations

Implementation Options

Option 1: Micrometer (Recommended)
Industry-standard metrics library with minimal overhead:

<dependency>
    <groupId>io.micrometer</groupId>
    <artifactId>micrometer-core</artifactId>
    <version>1.12.0</version>
</dependency>

// In NexusClient
private final MeterRegistry registry = new SimpleMeterRegistry();
private final Counter cacheHits = registry.counter("jnexus.cache.hits");
private final Counter cacheMisses = registry.counter("jnexus.cache.misses");

// On cache hit
cacheHits.increment();

// On cache miss
cacheMisses.increment();

Option 2: Dropwizard Metrics
Lightweight alternative:

<dependency>
    <groupId>io.dropwizard.metrics</groupId>
    <artifactId>metrics-core</artifactId>
    <version>4.2.25</version>
</dependency>

Option 3: Custom Metrics
Minimal implementation using AtomicLong for counters.

Export Options

For CLI:

• Print to console with --metrics flag
• Write to JSON file

For GUI:

• Display in status bar
• Add "Metrics" menu item

For Production:

• Export to Prometheus
• Export to StatsD
• Write to log file

Example CLI Output

$ ./jnexus.sh stats my-repo --metrics

Repository Statistics: my-repo
=====================================
Total Components: 1,234
Total Size:       15.2 GB

Performance Metrics:
=====================================
Cache Hit Rate:   87.3%
Avg Response Time: 245ms
HTTP Requests:    45
Retries:          2 (4.4%)
Components/sec:   27.4

Benefits

• Identify performance bottlenecks
• Measure cache effectiveness
• Track production usage patterns
• Inform optimization decisions
• Demonstrate value ("Saved X API calls via caching")

Overhead

• Micrometer: <1% performance impact
• Negligible memory footprint
• Can be disabled with flag

Priority

Low - Not blocking, but valuable for production deployments

Related

• Performance score: 88/100 (metrics would help identify improvement areas)
• Issue Code Quality: ReDoS prevention creates thread pool per validation - resource inefficient #53: Executor inefficiency (metrics would quantify impact)