Define modeling guidance for mismatches with standard env vars, add resource.attribute_list

[jack-berg]

Resolves open-telemetry/opentelemetry-specification#3966.

• Define modeling guidance that properties should use the most appropriate data types for the problem domain
• This may result in the lack of ability to support standard env vars where type mismatches occur
• When there is a type mismatch, a demonstrated need for platforms to contribute to config, and no reasonable alternative, we provide alternative versions of the property which match the standard env var
• For example, OTEL_RESOURCE_ATTRIBUTES is string whose value is a comma separated list of key value pairs. This is not a natural way to represent key value pairs in YAML, yet for reasons discussed in PR#3966 is important for platforms. Therefore, we provide an alternative .resource.attributes_list property which matches the format of OTEL_RESOURCE_ATTRIBUTES. Implementations merge the contents of .resource.attributes and .resource.attributes_list, with .resource.attributes taking priority.

[zeitlinger]

lgtm

[jsuereth]

FYI - I'd like to go a different direction with resource variables BUT I think this makes good progress on status quo.

See https://github.com/open-telemetry/oteps/pull/264/files - environment variable detector