Move Attributes to common location

[dyladan]

The specification now uses a common definition of Attributes for traces and metrics. Our definition of attributes should be moved from the trace directory to a common directory to reflect this.

[naseemkullah]

done? https://github.com/open-telemetry/opentelemetry-js-api/blob/main/src/common/Attributes.ts

[legendecas]

Fixed with #142.