[EPLB] Display the expert hotness comparison before and after eplb. (#6877)

### What this PR does / why we need it?
To intuitively show the effect of the eplb algorithm, we print the
expert heat before and after eplb.

### Does this PR introduce _any_ user-facing change?

### How was this patch tested?

![Snipaste_2026-02-28_17-23-42](https://github.com/user-attachments/assets/db1dadd1-cf96-44da-af34-57d41ccf412f)


- vLLM version: v0.16.0
- vLLM main:
15d76f74e2

Signed-off-by: shenchuxiaofugui <1311027364@qq.com>

docs/source/user_guide/feature_guide/eplb_swift_balancer.md

@@ -66,7 +66,7 @@ vllm serve Qwen/Qwen3-235B-A22 \
   --tensor-parallel-size 16 \
   --enable-expert-parallel \
   --additional-config '{
-    "expert_map_path": "/path/to/eplb.json"
+    "eplb_config": {"expert_map_path": "/path/to/eplb.json"}
   }'
 ```