Test metric aggregate start time complies with specification

[MrAlias]

The specification was updated here to have different start-time behavior for delta vs cumulative temporality for both synchronous and asynchronous instruments. Specifically:

The MetricReader selection of temporality as a function of instrument kind influences the starting timestamp (i.e. StartTimeUnixNano) of metrics data points received by MetricReader.Collect. For instruments with Cumulative aggregation temporality, successive data points received by successive calls to MetricReader.Collect MUST repeat the same starting timestamps (e.g. (T0, T1], (T0, T2], (T0, T3]). For instruments with Delta aggregation temporality, successive data points received by successive calls to MetricReader.Collect MUST advance the starting timestamp ( e.g. (T0, T1], (T1, T2], (T2, T3]). The ending timestamp (i.e. TimeUnixNano) MUST always be equal to time the metric data point took effect, which is equal to when MetricReader.Collect was invoked. These rules apply to all metrics, not just those whose point kinds includes an aggregation temporality field. See data model temporality for more details.

Our current testing only checks the presence of the start time. It does not test the correct cycling.

Test should be added to verify this and prevent regressions.