Offset or end-time option for getting metrics with latency

[dabest1]

gcpmetrics needs an option like offset or end-time for the metric's timestamp to handle latency in published metrics. Otherwise gcpmetrics comes back with an empty data frame when requesting a single data point if metrics have not been published by Google yet.

Here is an example project which uses offset to deal with latency in Google Stackdriver Metrics: https://github.com/frodenas/stackdriver_exporter