Implement Phase 9: Prometheus Metrics (v0.9.0)

- Add Prometheus metrics integration via wp-prometheus hooks
- Create Grafana dashboard JSON with 24 panels
- Add metrics settings tab with enable/disable toggle
- Expose inventory, booking, occupancy, revenue, and guest metrics

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

CLAUDE.md

@@ -935,3 +935,60 @@ Admin features always work; frontend requires valid license.
 - CSV export with BOM (`\xEF\xBB\xBF`) ensures Excel compatibility
 - Guest data aggregation from bookings uses unique key pattern for anonymous guests
 - Occupancy calculation: (booked nights / total room nights) * 100
+
+### 2026-02-03 - Version 0.9.0 (Prometheus Metrics)
+
+**Completed:**
+
+- Created `src/Integration/Prometheus.php` class (~700 lines)
+  - Integration with wp-prometheus via `wp_prometheus_collect_metrics` hook
+  - Dashboard registration via `wp_prometheus_register_dashboards` hook
+  - Option to enable/disable metrics collection
+  - Inventory metrics: buildings total, rooms by status, services by status
+  - Booking metrics: by status, check-ins/outs today, upcoming 7 days, avg duration
+  - Guest metrics: total, by status, repeat guests, new this month
+  - Occupancy metrics: current rate, monthly rate, occupied rooms, bed capacity
+  - Revenue metrics: this month, YTD, avg booking value, services revenue
+  - Optimized SQL queries using `$wpdb->prepare()` throughout
+- Created `assets/grafana/wp-bnb-dashboard.json` Grafana dashboard
+  - 24 panels with responsive grid layout
+  - Occupancy gauges with color-coded thresholds (red < 30%, orange < 50%, yellow < 70%, green ≥ 70%)
+  - Pie charts for bookings, rooms, and guests by status
+  - Revenue stat panels (this month, YTD, avg value, services)
+  - Guest stat panels (total, new, repeat, active services)
+  - Today's activity panels (check-ins, check-outs, upcoming)
+  - Prometheus datasource variable for flexibility
+  - Auto-refresh every 5 minutes
+- Updated `src/Plugin.php`
+  - Added Prometheus class import
+  - Initialized Prometheus integration in `init_components()`
+  - Added "Metrics" tab to settings page (6 tabs total)
+  - Added `render_metrics_settings()` method with WP Prometheus detection
+  - Added `save_metrics_settings()` method
+  - Metrics reference table showing all available metrics
+- Updated version to 0.9.0
+
+**Files Created:**
+
+- `src/Integration/Prometheus.php` - Prometheus metrics integration class
+- `assets/grafana/wp-bnb-dashboard.json` - Pre-configured Grafana dashboard
+
+**Files Changed:**
+
+- `src/Plugin.php` - Prometheus initialization, metrics settings tab
+- `wp-bnb.php` - Version bump to 0.9.0 (header and constant)
+- `CHANGELOG.md` - Added v0.9.0 release notes
+- `PLAN.md` - Marked Phase 9 as complete
+- `README.md` - Added Prometheus metrics documentation
+
+**Learnings:**
+
+- wp-prometheus uses `wp_prometheus_collect_metrics` action with collector object
+- Collector provides `register_gauge()` for fluctuating values
+- Labels are passed as array to `register_gauge()`, values to `set()`
+- Grafana dashboard JSON requires proper panel IDs and grid positions
+- Occupancy queries need careful date range handling for month boundaries
+- Revenue queries use `DECIMAL(10,2)` casting for accurate sums
+- Metrics should be cached or computed efficiently as they're scraped frequently
+- Dashboard registration requires file path, title, description, icon, and plugin name
+- Settings tab detection uses `$prometheus_active` to show WP Prometheus status