docs: add include_cost_in_streaming_usage docs snippet

docs/include_cost_in_streaming_usage.md

@@ -0,0 +1,54 @@
+## include_cost_in_streaming_usage
+
+Enable this setting to include the final request cost in streaming responses.
+
+Summary
+- Purpose: when `litellm_settings.include_cost_in_streaming_usage` is `true`, the proxy will inject the final usage/cost object into the last streaming chunk when the client requests `stream_options.include_usage: true`.
+
+How to enable
+
+In your proxy config (example):
+
+```yaml
+litellm_settings:
+  include_cost_in_streaming_usage: true
+```
+
+How to request usage in streaming calls
+
+When making a streaming request to a provider-compatible endpoint, set:
+
+```json
+{"stream_options": {"include_usage": true}}
+```
+
+Behavior notes
+- This setting only affects streaming responses when both the global flag (`litellm_settings.include_cost_in_streaming_usage`) and the per-request `stream_options.include_usage` are enabled.
+- For non-streaming requests the `x-litellm-response-cost` header remains available as before.
+- The injected usage object contains the same `usage.cost` breakdown used for non-streaming responses (input/output/cache/discounts) and will appear in the final stream chunk's `usage` field.
+
+Example final stream chunk (simplified):
+
+```json
+{
+  "id": "chunk-final",
+  "object": "chat.completion.chunk",
+  "choices": [],
+  "usage": {
+    "prompt_tokens": 12,
+    "completion_tokens": 6,
+    "cost": 0.000095,
+    "cost_breakdown": {
+      "input": 0.000060,
+      "output": 0.000035,
+      "discount": 0.000000
+    }
+  }
+}
+```
+
+Where to contribute
+- The public docs site (`litellm-docs`) hosts user-facing documentation. Add this snippet to the cost-tracking or streaming usage page in `litellm-docs` and link back to the `x-litellm-response-cost` docs if relevant.
+
+Notes for PR authors
+- Keep the entry short and add a link to the streaming usage examples/tests if needed. No code changes required in the core repo.