Investigate forcing data as stale

[dgl]

https://promcon.io/2017-munich/slides/staleness-in-prometheus-2-0.pdf
https://www.robustperception.io/staleness-and-promql

We could add staleness markers, e.g. if data is regularly arriving, add a marker after expected interval * 2... But this won't work for all metrics. Consider a scheme like geras_allowstale: as a prefix when querying?

Alternatively maybe this is inferring too much about the data we should force users to work this out via range queries and/or timestamp().