[Feature] Record metrics according to TimeOfMeasurement instead of Scrape Time

[philosowaffle]

Regarding timestamps we have:

1. The DateUtc provided by AmbientWeather, which should be the time the data was recorded at
2. The time the metric was scraped from ambientweather-local-server

Inherently, an Agent scraping prometheus metrics will store the time series data according to the scrape time. It assumes its getting a "live" reading of the current state of the metrics. This is how AWLS works today.

Depending on how much drift or delay there is between when AmbientWeather collected the metric and when it was scraped from the AWLS the metric data could be associated with a grossly incorrect timestamp.

Ideally, we would want our metrics associated with the time of measurement, not the scrape time. However, while prometheus does support writing a custom timestamp with a given metric, the library I'm using right now does not.

Need to investigate alternatives.

https://www.reddit.com/r/PrometheusMonitoring/comments/9h9olw/is_there_way_to_deliver_dataset_with_timestamps/
https://prometheus.io/docs/instrumenting/exposition_formats/