Stabilize metric start timestamp clarified mechanics

Tracking issue for stabilizing the [metric SDK star timestamps](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#start-timestamps) section, introduced in #4807, #4902.

Known prototypes:

* Java: https://github.com/open-telemetry/opentelemetry-java/pull/8180