[Caideyipi]

Summary

The DataNode at the supplied build exhausted its 4 GiB direct-memory limit while serving /metrics. The node had 240 subscription queues, whose paused WAL readers could each retain a direct segment buffer. This change releases those buffers between prefetch rounds while preserving reader positions and pending fragments, then reloads the current segment when a queue resumes. It also streams Prometheus snapshots as bounded 8 KiB UTF-8 chunks and reports EOF if a segment was shortened while its buffer was released.

Verification

• Targeted DataNode/grammar tests: 62 passed; one performance test was disabled by default.
• PrometheusReporterTest: 3 passed, including a response larger than 24 MiB under a 32 MiB direct-memory limit.
• Full reactor test-compile -P with-zh-locale: passed.
• The final EOF regression test could not be rerun locally: DataNode compilation fails on the existing generated relational parser/API mismatch (AstBuilder calls missing copyToStatement() methods). The full English compile encounters the same unrelated mismatch.