Add constants for message operations which are defined by spec.

[anuraaga]

No description provided.

[anuraaga]

I think this PR demonstrates that we need to split SemanticAttributes into multiple classes? Reasons

• attributes.setAttribute(MESSAGING_OPERATION, MESSAGING_OPERATION_SEND) is stuttery
• I don't see any way to automatically generate the current class based on the yaml files in a nice way

[thisthat]

• I don't see any way to automatically generate the current class based on the yaml files in a nice way

I'll open a draft PR to show how I would address the code generation. My idea is to provide an overload that accepts also enumerations as attribute values.

[jkwatson]

@anuraaga Yes, I'd like to have the values moved into their own class. Maybe SemanticConventionValues or some such.