I think the plugin needs to send zero values in some cases. Specifically, when the first observation (value) for metric{sampler_name="spam", failure="false"} is created, a matching observation metric{sampler_name="spam", failure="true"} 0 should be emitted.

Sending zero seems not a common thing in Prometheus, usually we send nothing to indicate that no observations have been made. That is correct for (for example) sampler_name because having observations on sampler spam does not imply that sampler ham exists.

However, assertion success and failure are related in the human mind: with a for assertions, we expect the total assertions a_t to be the sum of successes a_s and failures a_f. Unlike the samplers, having any number of successes implies that we also have failures (possibly 0), and vice versa.

Currently, the expectation is correct most of the time, iff a_s > 0 & a_f > 0. If no failures have been found yet: a_t != a_s + <undefined> The solution is to set a_f = 0 if a_s is set/modified and a_f has not been set yet.

Use case: I have a Grafana panel with the number of failures in my nightly tests, based on metric{failures="true"}. If all goes very well, the number shows up as "n/a" because no failures have been found. Switching to display the number of successes will go wrong when all tests fail.