CA-92154: Handle metrics files larger than 16 KiB

Unix.read doesn't read more than 16 KiB in one go. Hence use Unixext.really_read instead. This is potentially problematic particularly for reading the payload from the metrics file.

Signed-off-by: Jonathan Davies <jonathan.davies@citrix.com>
Merge pull request #1099 from jonludlam/CA-92154-clearwater

CA-92154: Handle metrics files larger than 16 KiB