## Bug description

When the `arize_phoenix` callback is configured to export traces over OTLP/gRPC and `PHOENIX_API_KEY` is set, LiteLLM successfully processes model requests but fails to export the resulting spans to Phoenix.

LiteLLM generates the authentication header as:

```text Authorization=Bearer <PHOENIX_API_KEY> ```

For OTLP/gRPC, this becomes gRPC metadata. gRPC metadata keys must be lowercase, so the exporter rejects the uppercase `Authorization` key.

The model request itself still returns HTTP 200. The failure only affects trace export and is logged asynchronously by the batch span exporter.

## Environment

- LiteLLM Proxy image: `ghcr.io/berriai/litellm-database:v1.93.0` - Python: 3.13 - Phoenix: self-hosted, authentication enabled - Export protocol: OTLP/gRPC - LiteLLM running in proxy mode

## Configuration

```yaml litellm_settings: callbacks: - arize_phoenix ```

Environment variables:

```bash PHOENIX_COLLECTOR_ENDPOINT=http://<phoenix-host>:4317 PHOENIX_API_KEY=<redacted> PHOENIX_PROJECT_NAME=<project-name> ```

The Phoenix endpoint is reachable from the LiteLLM container.

## Steps to reproduce

1. Run an authenticated self-hosted Phoenix instance wit…