Add http.client|server.request|response.size metrics

[jack-berg]

Implementation of newly added metric semantic conventions for http size.

Relies on HttpCommonAttributesGetter implementations returning non-null for requestContentLength and responseContentLength. Only a few implementations provide this data at the moment, but should be able to implement reasonably in more instrumentations by retrieving the Content-Length header value.

[jack-berg]

Open to suggestions for better names 🙂

[mateuszrzeszutek]

Only a few implementations provide this data at the moment, but should be able to implement reasonably in more instrumentations by retrieving the Content-Length header value.

We could even make that the default behavior -- the HTTP getters already expose the requestHeader() and responseHeader() methods, and we're already using it to calculate the http.host attribute.

What's interesting, I've checked all the implementations of these two methods, and the only instrumentation that extracts request/response sized and doesn't get them straight from the Content-Length header (at least on the first glance) is Armeria. I think I'd be fine with just dropping these methods altogether and just relying on the headers for everything.