Add OT semantic convention extensions to SpanAttribute

[slashmo]

As our Span & SpanAttribute are very much inspired by the OpenTelemetry spec, we might consider adding their semantic conventions as extensions to the Instrumentation library as well: https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/semantic_conventions/span-general.md

#62 (comment)

[pokryfka]

related with pokryfka/aws-xray-sdk-swift#22

Would be great to have type safe (enum or static string literals) Semantic conventions for HTTP spans

as well as some mappings / convenience methods from HTTP code to Span status code, see https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/semantic_conventions/http.md

example how it could be used in #95 (comment)

[slashmo]

I’m not sure yet where to put these extensions. I think they’re very valuable and should be added somewhere, but I don’t know if polluting the Instrumentation library is the best way to do it. What do you think about an additional library, e.g. OpenTelemetryInstrumentationSupport? CC @ktoso & @pokryfka

[ktoso]

A module OpenTelemetryInstrumentationSupport with extensions on the types sounds good to me 👍

Note that not just HTTP 😉 There's more things here: https://github.com/open-telemetry/opentelemetry-specification/tree/master/specification/trace/semantic_conventions