Clarify SDK must apply a default aggregation if not specified. (#2163)
Clarify SDK must apply a default aggregation if not specified. (#2163)

Co-authored-by: Reiley Yang <30492acf0f6ef8016cbaa2354294f909e563626a@microsoft.com>
Co-authored-by: Joshua MacDonald <f3bd74e7fbbcca8170a843c74945d604f45dfc0a@users.noreply.github.com>
open-telemetry/opentelemetry-specification,open-telemetry/opentelemetry-specification