Bigtable: adding 1ms to core exporter

[bshaffer]

in #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 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

[bshaffer]

removed this logic in d922602 because we implemented in nanos and the documentation says milliseconds, so something is off