Add sample metric YAML as fallback when OpenMetadata metrics unavailable

The /api/v1/metrics endpoint may not be available in all OpenMetadata instances.
This sample metric provides a fallback for demonstration purposes.

docs/metrics/revenue/total_revenue.yml

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+- name: total_revenue
+  display_name: Total Revenue
+  category: revenue
+  type: sum
+  unit: USD
+  grain: monthly
+  time_column: order_date
+  table: orders
+  expression: "SUM(total_amount)"
+  description: "Total revenue from all orders. Primary top-line metric tracking overall business performance across all channels and product categories."
+  dimensions:
+    - channel
+    - product_category
+    - region
+    - payment_method
+  notes:
+    - "Includes all completed orders, excludes cancelled and refunded"
+    - "Revenue is recognized at order completion date, not payment date"
+    - "Multi-currency orders are converted to USD at daily exchange rate"
+  synonyms:
+    - gross_revenue
+    - total_sales
+    - top_line_revenue
+  sql: |
+    SELECT
+        DATE_TRUNC('month', order_date) AS month,
+        SUM(total_amount) AS total_revenue
+    FROM orders
+    WHERE status = 'completed'
+    GROUP BY 1
+    ORDER BY 1
+  sql_by_channel: |
+    SELECT
+        DATE_TRUNC('month', order_date) AS month,
+        channel,
+        SUM(total_amount) AS revenue
+    FROM orders
+    WHERE status = 'completed'
+    GROUP BY 1, 2
+    ORDER BY 1, 3 DESC