Support development Attributes advisory metric option

[dashpole]

Problem Statement

From the Metrics API Spec:

Instrument advisory parameter: Attributes

Status: Development
Applies to all instrument types.
Attributes (a list of attribute keys) is the recommended set of attribute keys to be used for the resulting metrics.

Proposed Solution

Implement this in the metrics API without adding any public API surface. This will require introducing a pattern that allows defining experimental options outside of stable modules.