The format of /proc/diskstats is changing in linux-4.19 to include some
additional fields. See: https://www.kernel.org/doc/Documentation/iostats.txt
* collector/diskstats: use constants for some hard coded strings
* collector/diskstats: update diskstats for linux-4.19
* collector/diskstats: remove kernel doc url from individual metrics
Signed-off-by: Paul Gier <pgier@redhat.com>