Simplify prometheus metrics (#1981)

Co-authored-by: Mohit Reddy <mohitreddy1996@users.noreply.github.com>

python/sglang/srt/utils.py

@@ -781,6 +781,7 @@ def set_prometheus_multiproc_dir():
 
 
 def add_prometheus_middleware(app):
+    # We need to import prometheus_client after setting the env variable `PROMETHEUS_MULTIPROC_DIR`
     from prometheus_client import CollectorRegistry, make_asgi_app, multiprocess
 
     registry = CollectorRegistry()