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

[ericapisani]

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.

[linear-code]

PY-2242