Missing unit on lowest timestamp metric.

[SuperQ]

In #363 a new metric, prometheus_tsdb_lowest_timestamp, was added.

It seems like this is milliseconds, but is missing the unit.

Maybe we should convert this to seconds and make it prometheus_tsdb_lowest_timestamp_seconds.

[simonpasquier]

See #363 (comment) for the rationale. We have the same situation with the prometheus_tsdb_head_max_time and prometheus_tsdb_head_min_time metrics.

[SuperQ]

I don't think that rationale makes sense. If it's a timestamp, it has to have a unit, which we should know by the datatype.

[brian-brazil]

The unit is defined by the code that uses it though, not the tsdb. To the TSDB this is just a number.

[SuperQ]

Maybe the TSDB shouldn't expose this metric, but we have Prometheus create the metric instead so we have an opinion on what the type is.

[krasi-georgiev]

What are we doing with this one?