Pageview: Filter out null page_ids
Some records considered "pageview" don't have a page_id, but page_id is part of the Kafka key and needed by Trending, so we are filtering them out.
We are adding a counter to monitorize how many we lose by this situation.
We are also removing x_analytics and tls fields, they were big and redundant, this should reduce the amount of data produced.
Bug: T432204