Add support for TTL in views

[amuraru]

We've got an usecase where we have short/medium lived metrics that are current exposed to Prometheus. The metric itself makes sense for the duration of a job but then we'd need the metric to be removed from Prometheus. In OC words, we need a View to be unregistered if no data received for that View for X minutes.

[rakyll]

Is it possible to implement this outside of OC by keeping track of exporting views and their data? If so, I suggest implementing this feature outside of OC. Views are already complicated for average users, we should be careful about adding new functionality.