This looks like out of scope for TE, hence adding the respective label for it to  triage further.

Devtools attaches to the page as a debugger. Extensions can do it too: see chrome.debugger API documentation [1] and the debugger protocol documentation [2], specifically Network domain that allows extensions get the original network traffic data.

  [1]: https://developer.chrome.com/extensions/debugger
  [2]: https://chromedevtools.github.io/debugger-protocol-viewer/

chrome.debbuger is not an option since it requires user interaction.
I just want a small extension which counts traffic.

chrome.debugger requires user interaction? Did you test it?

It displays a small notification which is not acceptable for me.

This feature request may be considered a duplicate of issue 487422 "WebRequest API: allow extensions to read response body". Star that issue. If it'll be implemented you could easily do what you want.

It's not the same.
Response body can be unzipped.

I want the network traffic, not the size of request/response before compression.

Okay, so this issue expands on issue 487422 suggesting that the actually transmitted response size should also be exposed in the webRequest API. I think it's worth mentioning this nice idea in a comment to that main issue. 

I want also the request size, not only response.

Issue has not been modified or commented on in the last 365 days, please re-open or file a new bug if this is still an issue.

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