Add http content size semantic conventions

[XSAM]

Resolves #890

[evantorrie]

Don't know whether it needs to be in this PR, but we should look at including these in httptrace and othttp where appropriate.

[XSAM]

Since these attribute names already added to the semantic conventions, we should provide the predefined variable to standard package which is convenient for end-user, they can use the variable directly without writing it by hand.

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we should look at including these in httptrace and othttp where appropriate.

@evantorrie I add http.request_content_length to standard.httpBasicAttributesFromHTTPRequest, it will bring this variable to httptrace.Extract, othttp.Handler and othttp.Transport

[MrAlias]

Looks good outside of the const naming.

[MrAlias]

@evantorrie can you open an issue to address integrating these labels to the mentioned instrumentation packages if you think there is more work still?