Stale marker breaks metric

[codebien]

Unfortunately, @javaducky discovered that the latest version doesn't report accurate metrics. The issue seems generated from the latest added feature (stale marker).

For this reason, if the test is short, the time series cannot return any data point on a query. For example, using the Instant vector selector k6_http_reqs_total with some time frame and min step values it could not show the right total.

The suggestion is to roll back to v0.0.7 until a fix is available.

[codebien]

This is how Prometheus works we just need to document it very well. An alternative, specifically for Counters is to use last_over_time so in http_reqs example last_over_time(k6_http_reqs_total).