Use last_over_time to fix build info query #77
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When the version switches, we observed a problem where Prometheus would keep the value of the old gauge in memory and return it for the query using the build_info metric. This is a problem with our group_left query because Prometheus would complain that there are multiple matching label sets (the old and new version info).
Adding the last_over_time function seems to solve this because it seems to get rid of the old gauge value as soon as it no longer appears in the scrape.