## Problem

When LiteLLM proxy is deployed in a cascaded topology (one proxy forwarding to another), `metadata.tags` and `litellm_session_id` supplied by the caller are not forwarded to the downstream proxy. The downstream proxy sees requests without these fields, breaking tag-based routing, spend tracking by tag, and session correlation across the cascade.

## Proposed fix

In the LiteLLM-to-LiteLLM proxy forwarding path, propagate `metadata.tags` and `litellm_session_id` from the incoming request to the outgoing downstream request.

## Files

- `litellm/proxy/` (LiteLLM-to-LiteLLM forwarding logic)