one way you could verify here is to check the response payload and header on a test call or something to see if that information is contained within it. Since Dynatrace doesn't see it for your /repos/.../ then it very likely just does not exist there.
The response size is not always available in the HTTP headers. One such case is when the content is compressed, and typically delivered in "chunks". I can imagine that Dynatrace will not count all those chunks. The corresponding HTTP header is:
I am also interested in this topic.
I think that "Resopnse header size" and "Response body size" are no longer displayed in recent versions. Has anyone seen "Resopnse header size" or "Response body size" displayed?