Uncached I/O in Prometheus

Prometheus is introducing a feature called “use-uncached-io” to reduce its page cache footprint and improve memory predictability, particularly during disk compaction writes. By utilizing direct I/O on Linux, it bypasses the page cache for these writes, resulting in a 20-50% reduction in page cache usage without impacting CPU or disk I/O performance. While currently limited to compaction writes and Linux systems, the project welcomes contributions to expand coverage to more write paths, operating systems, and explore alternative approaches like `RWF_DONTCACHE`.