Generate semantic convention constants/classes

[aabmass]

Part of #932

open-telemetry/build-tools#5 and open-telemetry/opentelemetry-specification#571 introduce a tool that feeds semantic conventions (yaml files in spec repo) into a Jinja template to generate code. It would great if we could use this to generate constants for e.g. resource semantic conventions.

They have already used this in OT java to generate some things like https://github.com/heyams/opentelemetry-java/blob/master/api/src/main/java/io/opentelemetry/trace/attributes/SemanticAttributes.java

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Not needed for GA I don't think, but nice to have

[srikanthccv]

@aabmass Are you still looking into this? I can take a stab at this. I think this really helps in making instrumentations to follow semantic conventions.

[aabmass]

All yours

[srikanthccv]

@aabmass Where do you think these generated files should go? I can think of two options 1)Part of api(span attribute semantic conventions)/sdk(resource semconv) With this approach anybody taking dependency api/sdk can use Otel semantic conventions 2)Separate package in contrib repo (this seems good choice to me) .