`KstatSampler` could report missing self-stat samples

The KstatSampler object is used to publish oximeter statistics based on illumos kstats. It includes a few specific bits of information about its own inner workings, such as the number of samples dropped for each tracked object. It places those on a queue, from which oximeter collects data periodically. That's a tokio::sync::broadcast::Sender, which acts like a limited ringbuffer to avoid egregious memory consumption if oximeter doesn't come knocking frequently enough.

In that case, where we've missed samples, the sampler currently logs a message:

https://github.com/oxidecomputer/omicron/blob/f5b92b7cf1efcc2b4f3f8352d847ad0ffaaa9f11/oximeter/instruments/src/kstat/sampler.rs#L1319-L1326

It would be nice to record this as its own timeseries, something like kstat_sampler:self_stat_samples_dropped. This needs to be a generic number of self-stat samples we've dropped, not attached to any particular tracked target. At the point we get this error, we don't have the actual dropped Sample, only the count, so it's a general-purpose thing about the KstatSampler itself, not any target it tracks.

Start in oximeter/instruments/src/kstat/sampler.rs around lines 1319-1326, where dropped self-stat samples are currently logged. Trace how KstatSampler publishes its own statistics, then add a generic kstat_sampler:self_stat_samples_dropped timeseries using the dropped count rather than a tracked target. Done means the sampler records these missed self-stat samples as their own timeseries.