Support for timestamp in model

[djnym]

Hi, I have a statistics system mondemand and was working on a prometheus exporter (which I did here). I was going to also add a collector so that I could plug it in prometheus.erl but ran into an issue. The metric format for create_mf doesn't support the timestamp part of the exposition format. This is mostly an issue because of the way I keep summary statistics and quantiles in mondemand. Mondemand does reservoir sampling each minute, and when generating these statistics looks at the quantiles for the previous minute. So for my exporter I use the timestamp of the previous minute for summary stats and the current time for counters and gauges. If there were a way to forward the timestamp through the model and thus have it represented in the output, I could create a collector for mondemand. Anyway, just curious whether you've thought about this, whether it is a good idea, etc.