Add Prometheus metrics exporter

[willibrandon]

Description:
Export log event metrics to Prometheus for monitoring and alerting. This enables teams to monitor logging health and set up alerts based on log patterns.

Package: github.com/willibrandon/mtlog/adapters/prometheus

Proposed API:

import mtlogprom "github.com/willibrandon/mtlog/adapters/prometheus"

// Create metrics exporter
exporter := mtlogprom.NewExporter(logger,
    mtlogprom.WithCounters("log_events_total", "level", "source"),
    mtlogprom.WithHistogram("log_event_size_bytes"),
    mtlogprom.WithSinkMetrics(), // Track sink performance
)

// Expose at /metrics endpoint
http.Handle("/metrics", exporter.Handler())

// Or use as an enricher to add metrics
logger := mtlog.New(
    mtlog.WithConsole(),
    mtlog.WithEnricher(exporter),
)

Metrics to Export:

Event Metrics:

• log_events_total{level="error",source="api"} - Count by level and source
• log_event_size_bytes - Histogram of event sizes
• log_properties_per_event - Histogram of property counts

Performance Metrics:

• log_sink_latency_seconds{sink="seq"} - Sink write latency
• log_sink_errors_total{sink="elasticsearch",error="timeout"} - Sink errors
• log_sink_events_dropped_total{sink="file",reason="full"} - Dropped events

Template Cache Metrics:

• log_template_cache_hits_total - Cache hit count
• log_template_cache_misses_total - Cache miss count
• log_template_cache_size - Current cache size
• log_template_parse_duration_seconds - Template parsing time

Example Prometheus Queries:

# Alert on high error rate
rate(log_events_total{level="error"}[5m]) > 10

# Monitor sink performance
histogram_quantile(0.99, log_sink_latency_seconds)

# Track template cache efficiency
rate(log_template_cache_hits_total[5m]) / 
  (rate(log_template_cache_hits_total[5m]) + rate(log_template_cache_misses_total[5m]))

Implementation Notes:

• Use prometheus/client_golang
• Metrics should be zero-cost when not enabled
• Consider cardinality limits for labels
• Support custom metric namespaces
• Provide pre-built Grafana dashboard JSON