gateway: expose the eight telemetry headers requested in co-op/support#39

Adds x-litellm-model-name, -call-id, -key-spend, -response-duration-ms, -overhead-duration-ms, -callback-duration-ms, -attempted-retries and -attempted-fallbacks to the CORS expose list, on the GM's go-ahead. Renames COST_HEADERS to EXPOSED_HEADERS since the list is no longer only costs. x-litellm-key-spend discloses the caller's own key spend, which is their own data; noted in the thread so it can be dropped if anyone objects.

patch_expose_cost_headers.py

@@ -29,7 +29,7 @@ import sys
 
 # The header names the proxy can set, taken from the running image rather than
 # guessed: grep -rhoE 'x-litellm-[a-z0-9-]*cost[a-z0-9-]*' over the litellm package.
-COST_HEADERS = [
+EXPOSED_HEADERS = [
     "x-litellm-response-cost",
     "x-litellm-response-cost-input",
     "x-litellm-response-cost-output",
@@ -42,13 +42,23 @@ COST_HEADERS = [
     "x-litellm-response-cost-reasoning",
     "x-litellm-response-cost-tool-usage",
     "x-litellm-classifier-cost",
+    # Telemetry names, requested in co-op/support#39 (2026-10-31): model identity,
+    # call id, the caller's own key spend, timings and retry/fallback counts.
+    "x-litellm-model-name",
+    "x-litellm-call-id",
+    "x-litellm-key-spend",
+    "x-litellm-response-duration-ms",
+    "x-litellm-overhead-duration-ms",
+    "x-litellm-callback-duration-ms",
+    "x-litellm-attempted-retries",
+    "x-litellm-attempted-fallbacks",
 ]
 
 OLD = "    expose_headers=LITELLM_UI_ALLOW_HEADERS,\n"
 NEW = (
     "    expose_headers=LITELLM_UI_ALLOW_HEADERS\n"
     "    + [\n"
-    + "".join(f'        "{h}",\n' for h in COST_HEADERS)
+    + "".join(f'        "{h}",\n' for h in EXPOSED_HEADERS)
     + "    ],\n"
 )
 
@@ -72,7 +82,7 @@ def main() -> int:
         return 0
 
     target.write_text(src.replace(OLD, NEW))
-    print(f"expose-cost-headers: appended {len(COST_HEADERS)} cost header names to expose_headers in {target}")
+    print(f"expose-cost-headers: appended {len(EXPOSED_HEADERS)} cost header names to expose_headers in {target}")
     return 0