Bigtable: adding 1ms to core exporter

in https://github.com/googleapis/google-cloud-node/pull/9059 we set `endTime` to `startTime + 1ms` if `endTime` is less than or equal to `startTime` in the bigtable metric exporter. This is logic that also shows up in the [go opentelemetry exporter](https://github.com/GoogleCloudPlatform/opentelemetry-operations-go/blob/07d0107d618a4eec8f7866a3f8e86c71497655cf/exporter/metric/metric.go#L684) to satisfy the Monitoring API's requirement that the `endTime` is at least 1ms after `startTime` for time intervals: https://cloud.google.com/monitoring/api/ref_v3/rpc/google.monitoring.v3#timeinterval

> The start time of a new interval must be at least a millisecond after the end time of the previous interval because intervals are closed. If the start time of a new interval is the same as the end time of the previous interval, then data written at the new start time could overwrite data written at the previous end time.

This issue is to track this change to make sure it does not have any adverse affects while we roll it out in #9059

Start by reviewing PR #9059 and the Bigtable metric exporter change, then compare its endTime behavior with the Monitoring API time-interval requirement. Done means confirming the rollout has no adverse effects; the issue does not name tests or files.

Written by the indexing model from the issue text.