[change] Reduced network calls in device metric flow #501

[pandafy]

Closes #501

Checks:

• [ ] I have manually tested the proposed changes
• [ ] I have written new test cases to avoid regressions (if necessary)
• [ ] I have updated the documentation (e.g. README.rst)

[pandafy]

The CI is failing due to a mismatch in formatting of timestamp. In the current implementation (master branch), the datetime object gets serialized when passed to celery. Due to this, the time argument of post_metric_write signal is a string. When using UDP for writing timeseries database, the datetime object is not serialized (because the task is executed synchonously) and the formatting of timestamp differs.

Moreover, if we pass a timestamp string to Metric.write method, the post_metric_write signal will use the exact string for time argument which leads to inconsistent results.

I think we should format all incoming timestamps to the ISO format in Metric.write which also includes the timezone information.