Stabilize Prometheus Exporter: Default Aggregation

As part of the [initiative](https://github.com/open-telemetry/opentelemetry-specification/issues/4805) to stabilize the Prometheus Exporter spec, the Python SDK's Prometheus Exporter should introduce support for [Default Aggregation](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk_exporters/prometheus.md#default-aggregation) configuration to align with the spec. The current implementation status is documented in https://github.com/open-telemetry/opentelemetry-specification/issues/4986#issuecomment-4214660688.