Fix timestamp handling for the lastvalue aggregation (open-telemetry#5517)

Fixes open-telemetry#5510

From https://opentelemetry.io/docs/specs/otel/metrics/sdk/#metricreader:

> 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](https://opentelemetry.io/docs/specs/otel/metrics/sdk/#collect)
was invoked. These rules apply to all metrics, not just those whose
[point
kinds](https://opentelemetry.io/docs/specs/otel/metrics/data-model/#point-kinds)
includes an aggregation temporality field.

Before open-telemetry#5305, we
used the measurement time as the timestamp, but it didn't matter because
the collection time is always the same as the measurement time for
asynchronous instruments. We didn't catch the issue when we implemented
synchronous instruments.

This PR changes the (end) timestamp handling for the Last Value
aggregation to match the (end) timestamp handling for sum and histogram
aggregations. As described in the spec above, we take the timestamp when
computing the aggregation during Collect.

Author: dashpole

CHANGELOG.md

@@ -45,6 +45,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - Run the `Detect` method in `go.opentelemetry.io/otel/sdk/resource` in parallel. (#5402)
 - Fix panic in baggage creation when a member contains 0x80 char in key or value. (#5494)
 - Correct comments for the priority of the `WithEndpoint` and `WithEndpointURL` options and their coresponding environment variables in  in `go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracegrpc`. (#5508)
+- Fix stale timestamps reported by the lastvalue aggregation. (#5517)
 
 ## [1.27.0/0.49.0/0.3.0] 2024-05-21