docs cost tracking by api key

docs/my-website/docs/proxy/cost_tracking.md

@@ -117,6 +117,8 @@ That's IT. Now Verify your spend was tracked
 <Tabs>
 <TabItem value="curl" label="Response Headers">
 
+Expect to see `x-litellm-response-cost` in the response headers with calculated cost
+
 <Image img={require('../../img/response_cost_img.png')} />
 
 </TabItem>
@@ -337,6 +339,61 @@ curl -X GET 'http://localhost:4000/global/spend/report?start_date=2024-04-01&end
 ```
 
 
+</TabItem>
+
+<TabItem value="per key" label="Spend Per API Key">
+
+
+👉 Key Change: Specify `group_by=api_key`
+
+
+```shell
+curl -X GET 'http://localhost:4000/global/spend/report?start_date=2024-04-01&end_date=2024-06-30&group_by=api_key' \
+  -H 'Authorization: Bearer sk-1234'
+```
+
+##### Example Response
+
+
+```shell
+[
+  {
+    "api_key": "ad64768847d05d978d62f623d872bff0f9616cc14b9c1e651c84d14fe3b9f539",
+    "total_cost": 0.0002157,
+    "total_input_tokens": 45.0,
+    "total_output_tokens": 1375.0,
+    "model_details": [
+      {
+        "model": "gpt-3.5-turbo",
+        "total_cost": 0.0001095,
+        "total_input_tokens": 9,
+        "total_output_tokens": 70
+      },
+      {
+        "model": "llama3-8b-8192",
+        "total_cost": 0.0001062,
+        "total_input_tokens": 36,
+        "total_output_tokens": 1305
+      }
+    ]
+  },
+  {
+    "api_key": "88dc28d0f030c55ed4ab77ed8faf098196cb1c05df778539800c9f1243fe6b4b",
+    "total_cost": 0.00012924,
+    "total_input_tokens": 36.0,
+    "total_output_tokens": 1593.0,
+    "model_details": [
+      {
+        "model": "llama3-8b-8192",
+        "total_cost": 0.00012924,
+        "total_input_tokens": 36,
+        "total_output_tokens": 1593
+      }
+    ]
+  }
+]
+```
+
 </TabItem>
 
 </Tabs>