Measure bandwidth used by web socket in React Native app

In our application, staff use their phones to log activities within a business. They end up using 0.5GB-2GB data per month on average.

I'm trying to build functionality into our app that logs data usage so that we can send it back to the business in the form of an expense claim.

In the example code below, how can I determine how much bandwidth/data was used by the device sending the message over a WebSocket?

var ws = new WebSocket('ws://host.com/path');

ws.onopen = () => {
  ws.send('something');
};

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