After using a volumetric flow meter and a densitometer to measure the mass flow rate, is it usually still necessary to install an on-line gas analyzer?

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In current long-distance gas pipeline systems, are the gas analyzers installed on the main pipelines in an online manner or is sampling done periodically? Especially at the first station, is it true that SCADA systems are usually located near that station? :) Also, regarding the impurities in natural gas, such as carbon dioxide, hydrogen sulfide, and moisture, is it necessary to remove these amounts by measuring their components during commercial measurement? Natural gas is currently more likely to be measured in terms of energy; the useful portion is determined by its calorific value, while impurities do not have any impact. However, for some projects, mass flow rate may be used as the unit for settlement. In such cases, is the mass of impurities removed before settlement, or is the price already fixed based on natural gas containing a certain amount of impurities, without the need to remove those impurities further? Or, when the amount of impurities is below a certain percentage, it is the user who has to cover the cost; otherwise, it is the energy supplier who must remove that amount before settling the bill.