feat(otlp): Infer span description for `db` spans

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This PR is part of the OTLP initiative and will allow Relay to determine the description field on spans by using the provided metadata. In this case specifically, Relay will use check if the span's category is set to db, in which case it will find the DB query and set that as the span's description.

Co-authored-by: Matt Quinn [email protected]`