Instrument `pydantic_ai.embeddings.Embedder` with `gen_ai.embeddings` spans

`Embedder.embed()`, `embed_query()`, and `embed_documents()` (and their sync variants) make model calls with no spans today.

OTel defines `gen_ai.embeddings` as a valid `gen_ai.operation.name` and we already have `OP.GEN_AI_EMBEDDINGS` defined in `sentry_sdk/consts.py`. Should create `gen_ai.embeddings` spans with model name, system, and token usage attributes.