feat: add summary notes changes

Signed-off-by: Manan Gupta <[email protected]>

changelog/21.0/21.0.0/summary.md

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+
+## Summary
+
+### Table of Contents
+
+- **[Major Changes](#major-changes)**
+  - **[Deletions](#deletions)** 
+    - [Deletion of deprecated metrics](#metric-deletion)
+  - **[Breaking changes](#breaking-changes)**
+
+## <a id="major-changes"/>Major Changes
+
+### <a id="deletions"/>Deletion
+
+#### <a id="metric-deletion"/>Deletion of deprecated metrics
+
+The following metrics that were deprecated in the previous release, have now been deleted.
+
+
+|                 Metric Name                  |
+|:--------------------------------------------:|
+|           `analysis.change.write`            |        
+|                `audit.write`                 |     
+|            `discoveries.attempt`             |          
+|              `discoveries.fail`              |        
+| `discoveries.instance_poll_seconds_exceeded` | 
+|          `discoveries.queue_length`          |       
+|          `discoveries.recent_count`          |        
+|               `instance.read`                |            
+|           `instance.read_topology`           |       
+|         `emergency_reparent_counts`          |       
+|          `planned_reparent_counts`           |      
+|      `reparent_shard_operation_timings`      |  
+		
+
+

changelog/21.0/README.md

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+## v21.0
+* **[21.0.0](21.0.0)**