Support OpenMetrics _created timestamp in Prometheus

The proposal consists in treating the time series creation time as another metadata. We propose a few incremental improvements that can be released separately, and therefore faster. We start by dropping time series when negotiating OpenMetrics format. Then changing the current metadata behavior to be correlated with MetricPoint instead of MetricFamily. The third step is storing the last seen value of _created for each MetricPoint. Lastly, storing a symbolic value each time the _created timestamp minus the scrape timestamp is lower than the scrape interval.