feat: Add custom metric builder, export/import, and Grafana dashboards (v0.3.0)

- Custom Metrics Builder with admin UI for gauge metrics
- Support for static values and WordPress option-based values
- JSON-based export/import with skip/overwrite/rename modes
- Three Grafana dashboard templates (overview, runtime, WooCommerce)
- New tabs: Custom Metrics and Dashboards
- Reset runtime metrics button

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

src/Installer.php

@@ -64,6 +64,7 @@ final class Installer {
             'wp_prometheus_enable_default_metrics',
             'wp_prometheus_enabled_metrics',
             'wp_prometheus_runtime_metrics',
+            'wp_prometheus_custom_metrics',
         );
 
         foreach ( $options as $option ) {