Unify Attributes and Labels

[MrAlias]

Currently Attributes in the tracing portion of this code is synonymous with label.KeyValue. This duplicate naming (attribute and label) will lead to confusion for new (and likely existing) users. This duplication comes from the specification itself unfortunately. Attributes are used in the tracing and resource specification (and have been unified in the "common" part of the specification) and labels are used in the metric specification. There was a previous attempt to resolve this difference in the specification, but it did not produce a resolution.

Can we do better in this repository?

[seh]

Since we are not afraid of renaming packages and moving types, have we considered a package name like "attr"?

[MrAlias]

My hope was to unify on the term label as it is a complete word and shorter than attribute.

It is important that this work does not impact the compatibility of the project. That means that the Spant field will likely need to remain being named Attributes. This might also apply to the Resource, but since it is only used in the internal SDK, that might not be as strong a requirement.

[MrAlias]

open-telemetry/opentelemetry-specification#948

This is likely going to be the top issue at the next Specification SIG meeting.

[MrAlias]

Specification issue is being tracked here now: open-telemetry/opentelemetry-specification#1113

[MrAlias]

Given the label package is already used for attributes, we should not be blocked with issues blocking open-telemetry/opentelemetry-specification#1113 and should be able rename the label package and metric use of it.

[MrAlias]

Let's rename the label package to attr.