huggingface_hub: Instrument feature_extraction (embeddings)

`InferenceClient.feature_extraction()` is the HuggingFace equivalent of embeddings. The SDK has `OP.GEN_AI_EMBEDDINGS` defined and both OpenAI and Cohere instrument their embedding methods. This is a meaningful gap for users who use HF for embeddings. Should cover both sync `InferenceClient` and async `AsyncInferenceClient`.