err-mimir-sample-timestamp-too-old event logs don't match cortex_discarded_samples_total label

[pr00se]

sample-out-of-bounds error logs were changed to say sample-timestamp-too-old, but the same events are exposed as part of the cortex_discarded_samples_total metric with the old sample-out-of-bounds label. This leads to confusion when trying to tie metrics and logs together, unless you intrinsically know to do this translation.

We should change the metric label to match the logs, as that is the preferred phrasing for this type of discard.