feat: add Prometheus alerting rules example

[abtreece]

Summary

• Add pre-built Prometheus alerting rules (examples/prometheus-alerts.yml)
• Update examples/prometheus.yml to include rule_files configuration
• Update README with alerting configuration section
• Update CHANGELOG

Alert Groups

Group	Alerts	Description
nginx_availability	NginxDown, NginxNoRequests	Instance health
nginx_errors	NginxHighErrorRate, NginxElevatedErrorRate, NginxHigh4xxRate	Error rate thresholds
nginx_latency	NginxHighLatencyP99, NginxElevatedLatencyP99, NginxHighAverageLatency	Latency percentiles
nginx_upstream	NginxUpstreamFailures, NginxUpstreamHighErrorRate, NginxUpstreamHighLatency, NginxUpstreamNoRequests	Backend health
nginx_connections	NginxHighConnections, NginxConnectionsNotHandled	Connection limits
nginx_cache	NginxLowCacheHitRate, NginxHighCacheBypass	Cache efficiency
nginx_rate_limiting	NginxHighRateLimitRejections, NginxRateLimitingActive	Rate limit status
nginx_ssl	NginxLowSSLSessionReuse, NginxDeprecatedTLSProtocol	SSL/TLS health
nginx_traffic	NginxTrafficSpike, NginxTrafficDrop, NginxHighBandwidth	Traffic anomalies

Severity Levels

• critical - Immediate action required (NGINX down, high 5xx rate, upstream failures)
• warning - Investigation needed (elevated latency, low cache hit rate)
• info - Informational (rate limiting active)

Test plan

• Validate YAML syntax with promtool check rules prometheus-alerts.yml
• Import rules into Prometheus
• Verify alerts fire correctly with test traffic

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR adds comprehensive Prometheus alerting rules for NGINX monitoring with production-ready thresholds. The changes enable operators to receive notifications for critical issues like service downtime, error spikes, latency problems, and upstream failures.

Changes:

• Adds examples/prometheus-alerts.yml with 26 alerting rules organized into 9 groups covering availability, errors, latency, upstream health, connections, caching, rate limiting, SSL/TLS, and traffic anomalies
• Updates examples/prometheus.yml to reference the new alert rules file and includes commented alertmanager configuration
• Documents the alerting configuration in README with a summary of alert groups

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
examples/prometheus.yml	Adds rule_files configuration pointing to prometheus-alerts.yml and commented alertmanager setup
examples/prometheus-alerts.yml	New file containing 26 alerting rules across 9 groups with critical, warning, and info severity levels
README.md	Adds "Configure Alerting" section documenting the alert groups
CHANGELOG.md	Documents the new alerting rules feature in the Unreleased section

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