fix: Final-annotate the per-boot dir pick and test the exit hook

main's type-discipline gate counts assignments without a Final declaration
against the merge-base ceiling, and unpacking targets cannot carry Final:
the second `multiproc_dir, action = ...` pick re-binds both names (LIT010).
Declare both as Final and defer the assignment to the branch that picks the
value. The creator-PID exit hook becomes a staticmethod so the
creator-only-removes branch is unit-covered, which also lifts patch
coverage back above target.

litellm/proxy/proxy_cli.py

@@ -4,6 +4,7 @@ import json
 import os
 import random
 import re
+import shutil
 import subprocess
 import sys
 import urllib.parse as urlparse
@@ -712,7 +713,6 @@ class ProxyInitializationHelpers:
         with prometheus configured as a callback in config.yaml, or the separate metrics server (always, since
         callbacks may also be enabled from the DB after startup).
         """
-        import shutil
         import tempfile
 
         from litellm.proxy.prometheus_cleanup import wipe_directory
@@ -725,26 +725,24 @@ class ProxyInitializationHelpers:
                 wipe_directory(configured_dir)
             return None
 
+        multiproc_dir: Final[str]
+        action: Final[str]
         if configured_dir:
             # An operator-set directory is theirs to manage; keep the historical wipe-on-boot.
             os.makedirs(configured_dir, exist_ok=True)
             wipe_directory(configured_dir)
-            multiproc_dir, action = configured_dir, "Using existing"
+            multiproc_dir = configured_dir
+            action = "Using existing"
         else:
             # Unique per boot: a fixed shared name lets a second proxy on this host wipe the first one's counters.
             import atexit
 
-            multiproc_dir, action = tempfile.mkdtemp(prefix="litellm_prometheus_multiproc_"), "Auto-created"
+            multiproc_dir = tempfile.mkdtemp(prefix="litellm_prometheus_multiproc_")
+            action = "Auto-created"
 
             # Only the creating process removes the dir on exit. Forked workers share it and
             # must not delete samples their siblings are still writing.
-            creator_pid = os.getpid()
-
-            def _cleanup_auto_created_dir() -> None:
-                if os.getpid() == creator_pid:
-                    shutil.rmtree(multiproc_dir, ignore_errors=True)
-
-            atexit.register(_cleanup_auto_created_dir)
+            atexit.register(ProxyInitializationHelpers._rmtree_auto_created_dir, multiproc_dir, os.getpid())
 
         os.environ["PROMETHEUS_MULTIPROC_DIR"] = multiproc_dir
         print(f"LiteLLM: {action} PROMETHEUS_MULTIPROC_DIR={multiproc_dir}")
@@ -752,6 +750,13 @@ class ProxyInitializationHelpers:
 
     _maybe_setup_prometheus_multiproc_dir = maybe_setup_prometheus_multiproc_dir
 
+    @staticmethod
+    def _rmtree_auto_created_dir(multiproc_dir: str, creator_pid: int) -> None:
+        """atexit hook for a boot-auto-created PROMETHEUS_MULTIPROC_DIR: only the creating process
+        removes the dir, so forked workers sharing it never delete samples their siblings still write."""
+        if os.getpid() == creator_pid:
+            shutil.rmtree(multiproc_dir, ignore_errors=True)
+
 
 @click.command()
 @click.argument("cli_args", nargs=-1)

tests/unit/proxy/test_prometheus_cleanup.py

@@ -292,8 +292,15 @@ class TestMaybeSetupPrometheusMultiprocDir:
             assert os.path.exists(stale)
 
             os.environ.pop("PROMETHEUS_MULTIPROC_DIR", None)
+
+            # The exit hook removes the dir only from the creating process: a forked worker
+            # (different PID) sharing the dir must leave its siblings' samples alone.
+            ProxyInitializationHelpers._rmtree_auto_created_dir(first, 999999)
+            assert os.path.exists(first)
+            ProxyInitializationHelpers._rmtree_auto_created_dir(second, os.getpid())
+            assert not os.path.exists(second)
+
             shutil.rmtree(first, ignore_errors=True)
-            shutil.rmtree(second, ignore_errors=True)
 
     def test_configured_dir_is_still_wiped_on_boot(self, tmp_path):
         """A directory the operator set keeps its historical wipe-on-boot lifecycle."""