Add Prometheus Pushgateway and data ingestion tool

This commit adds a complete Prometheus data ingestion solution:

1. Pushgateway Helm Chart (f3s/pushgateway/)
   - Standalone helm chart for Prometheus Pushgateway
   - Deployed to monitoring namespace
   - Receives pushed metrics via HTTP POST on port 9091

2. Prometheus Pusher (f3s/prometheus-pusher/)
   - Standalone Go binary (12MB) for pushing metrics to Pushgateway
   - Demonstrates all Prometheus metric types:
     * Counter (app_requests_total)
     * Gauge (app_active_connections, app_temperature_celsius)
     * Histogram (app_request_duration_seconds)
     * Labeled Counter (app_jobs_processed_total)
   - Pushes metrics every 15 seconds
   - Includes comprehensive documentation and examples

3. Prometheus Configuration
   - Updated additional-scrape-configs.yaml to scrape Pushgateway
   - Uses honor_labels to preserve pushed metric labels

Architecture:
  Go Binary → Pushgateway → Prometheus → Grafana

The pusher binary generates realistic example metrics and pushes them
to Pushgateway in Prometheus text format. Prometheus then scrapes the
Pushgateway and makes the metrics available for querying and alerting.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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