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/Metrics/Collector.php

@@ -10,6 +10,7 @@ namespace Magdev\WpPrometheus\Metrics;
 use Prometheus\CollectorRegistry;
 use Prometheus\Storage\InMemory;
 use Prometheus\RenderTextFormat;
+use Magdev\WpPrometheus\Metrics\CustomMetricBuilder;
 
 // Prevent direct file access.
 if ( ! defined( 'ABSPATH' ) ) {
@@ -113,6 +114,10 @@ class Collector {
         // Collect runtime metrics (HTTP requests, DB queries).
         $this->collect_runtime_metrics( $enabled_metrics );
 
+        // Collect custom user-defined metrics.
+        $custom_builder = new CustomMetricBuilder();
+        $custom_builder->register_with_collector( $this );
+
         /**
          * Fires after default metrics are collected.
          *