Add support for accepting UNIX timestamps to get_metric_data_points

`cloud_monitoring.get_metric_data_points` operates on a start and end timestamp, but previously the function was unable to directly receive a UNIX timestamp value, only Python date/datetime or string-based timestamps. `utils.to_timestamp` now returns numbers given to it, as they're already timestamps. This change makes it work as the docstring suggests.

Also added a comment to `get_metric_data_points` to explain the existing conversion of UNIX timestamps to integers. The API currently times out when given timestamps with fractional seconds.