Stackdriver metrics timestamp sometimes goes backwards in time

stackdriver-exporter 0.9.0 introduced timestamps associated with each exported series. When doing experiments, in some cases (in our case stackdriver_l_7_lb_rule_logging_googleapis_com-metrics) the timestamp jumped as much as 2 minutes into the past (and return back to the present). When fetching the metrics every ~2seconds, the timestamp differences were as follows:

2020-06-16T17:15:59+02:00  2084
2020-06-16T17:16:01+02:00  -57920
2020-06-16T17:16:03+02:00  1878
2020-06-16T17:16:05+02:00  2023
2020-06-16T17:16:07+02:00  1972
2020-06-16T17:16:09+02:00  1941
2020-06-16T17:16:11+02:00  1940
2020-06-16T17:16:13+02:00  62148
2020-06-16T17:16:15+02:00  1954
2020-06-16T17:16:17+02:00  2026
2020-06-16T17:16:19+02:00  1904
2020-06-16T17:16:21+02:00  2252
2020-06-16T17:16:23+02:00  1924

This doesn't work well with Prometheus, since any data that appear from the past will get dropped.

Start by reproducing the timestamp behavior described for stackdriver_l_7_lb_rule_logging_googleapis_com while fetching metrics every two seconds, then trace where exported series timestamps are assigned. Done means exported timestamps no longer jump into the past and the resulting data is accepted correctly by Prometheus.