sdk/metrics: _LastValueAggregation and OTLP encoders omit start_time_unix_nano for Gauge #5849

Per the MetricReader specification and Start timestamps specification, StartTimeUnixNano rules apply to all metrics according to the MetricReader's configured temporality for the instrument kind, "not just those whose point kinds includes an aggregation temporality field."

Currently, Gauge and ObservableGauge (LastValue aggregation) omit start_time_unix_nano:

• DefaultAggregation._create_aggregation() (aggregation.py#L1176-L1188) and LastValueAggregation._create_aggregation() (aggregation.py#L1330-L1341) do not pass start_time_unix_nano to _LastValueAggregation, and _LastValueAggregation.collect() (aggregation.py#L367-L410) always sets start_time_unix_nano=None.
• The OTLP protobuf encoder (proto/common/_internal/metrics_encoder/__init__.py#L222-L233) and OTLP JSON encoder (json/common/_internal/metrics_encoder/__init__.py#L261-L273) also omit start_time_unix_nano=data_point.start_time_unix_nano when encoding Gauge data points.