lustre_exporter.go: Add scrape_durations_seconds
lustre_exporter.go: Add scrape_durations_seconds

Adding support to track duration of scrape jobs in the future

Signed-Off-By: Joe Handzik <91435a5354fd1941fa818cc717e722f572f40c0d@hpe.com>

HewlettPackard/lustre_exporter,roclark/lustre_exporter