Update aggregation_temporality to be an enum

[kaylareopelle]

The aggregation_temporality option in the Sum and ExplicitBucketHistogram aggregations is currently open-ended. The only spec'd options for this are :delta and :cumulative. Let's update the assignment to use an enum instead.

Relates to #1555

[github-actions]

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[kaylareopelle]

@xuan-cao-swi - I looked into this a little more after the SIG. The class I was thinking of as an example is is OpenTelemetry::Trace::Status. In the spec, the Set Status API only has three valid options. The class is used like this:

opentelemetry-ruby/api/lib/opentelemetry/trace/tracer.rb

Line 40 in e654d73

span&.status = Status.error("Unhandled exception of type: #{e.class}")

The class method is always called to get a valid status.

@arielvalentin, is something like this what you were initially thinking of when you recommended an enum for aggregation temporality or did you have something else in mind?

[xuan-cao-swi]

Thanks for the example! I'd like to contribute on this based on the structure of OpenTelemetry::Trace::Status.