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* Improve plugin/cache metrics * Add coredns_cache_prefetch_total metric to track number of prefetches. * Remove unnecessary Cache.get() call which would incorrectly increment cache counters. * Initialize all counters and gauges at zero. * Allow prefetching of a single request per ttl The original implementation didn't allow prefetching queries which are only requested once during the duration of a TTL. The minimum amount of queries which had to be seen was therefore capped at 2. This change also implements a real prefetch test. The existing test was a noop and always passed regardless of any prefetch implementation. * Fix prefetching for items with a short TTL The default prefetch threshold (percentage) is 10% of the lifetime of a cache item. With the previous implementation, this disabled prefetching for all items with a TTL < 10s (the resulting percentage would be 0, at which point a cached item is already discarded). This change uses a time based threshold calculation and ensures that a prefetch is triggered at a TTL of 1 at the latest. * Fix wrong duration reporting of cached responses The logging and metrics plugins (among others) included the duration of a cache prefetch in the request latency of client request. This change fixes this wrong reporting and executes the prefetch request in a goroutine in the background.