Add HTTP semantic convention for content size

[dyladan]

open-telemetry/opentelemetry-specification#641

This will need to be added to the HTTP and HTTPS plugins, as well as to the Semantic Conventions module.

[michaelgoin]

Per meeting, I can grab this.

[dyladan]

Oops looks like i assigned @obecny instead of @michaelgoin

[carlo-808]

Greetings, I work with @michaelgoin at New Relic and he and I both decided I could take this issue to get more familiar with the project. Can someone assign me this issue? I'm looking forward to contributing to the project. Thanks!

[carlo-808]

@dyladan I have some questions about this issue. Should we be calculating the value for request_content_length or just use the content-length header value to populate request_content_length? For request_content_length_uncompressed, do we populate this value with the content-length header if no transport encoding is used OR should it be calculated as well?

[dyladan]

For request_content_length, simply using the header value should be fine. I'm not that familiar with the available headers but I think uncompressed will need to be calculated.

[nijotz]

PR #1625 opened to add this feature