Felt like I had to dig for too long to figure this out so hopefully it can help someone else.

With BigQuery, if you are trying to insert a field of type RECORD and are getting this message your map needs to be inside brackets.

Like this:

row_json =
  %{
    "timestamp" => bq_timestamp,
    "event_message" => payload.log_message,
    "metadata" => [payload.metadata]
  }

Not like this:

row_json = 
  %{
    "timestamp" => bq_timestamp,
    "event_message" => payload.log_message,
    "metadata" => payload.metadata
  }