[model-gateway] Add Redis support as a history backend (#16300)

docs/advanced_features/sgl_model_gateway.md

@@ -858,6 +858,7 @@ Prefill pods can expose bootstrap ports via the `sglang.ai/bootstrap-port` annot
 | `none` | No persistence | `--history-backend none` |
 | `oracle` | Oracle Autonomous Database | `--history-backend oracle` |
 | `postgres` | PostgreSQL Database | `--history-backend postgres` |
+| `redis` | Redis | `--history-backend redis` |
 
 ### Oracle Configuration
 
@@ -892,6 +893,22 @@ python -m sglang_router.launch_router \
   --history-backend postgres
 ```
 
+### Redis Configuration
+
+```bash
+export REDIS_URL="redis://localhost:6379"
+export REDIS_POOL_MAX=16
+export REDIS_RETENTION_DAYS=30
+
+python -m sglang_router.launch_router \
+  --backend openai \
+  --worker-urls https://api.openai.com \
+  --history-backend redis \
+  --redis-retention-days 30
+```
+
+Use `--redis-retention-days -1` for persistent storage (default is 30 days).
+
 ---
 
 ## WASM Middleware