* feat(litellm_pre_call_utils.py): support `x-litellm-tags` request header
allow tag based routing + spend tracking via request headers
* docs(request_headers.md): document new `x-litellm-tags` for tag based routing and spend tracking
* docs(tag_routing.md): add to docs
* fix(utils.py): only pass str values for openai metadata param
* fix(utils.py): drop non-str values for metadata param to openai
preview-feature, otel span was being sent in