Allow regenerating the LTTng-UST and LTTng-modules metadata after NTP major adjustment
During an active non-live session (kernel and UST per-uid), the "lttng metadata regenerate" can be issued at any time to recreate the metadata file(s) either locally or through a lttng-relayd in case of streaming. As usual with any session, the guarantee for this file to be commited on disk is after the "lttng stop" command.
When the metadata is regenerated, the NTP offset is recomputed, so if a major NTP adjustment was made after the session was started, the offset will be fixed to reflect this adjustment.
Since this offset is a best-effort approximation, we might notice a slight variation (less than 10ns in our tests) of the offset even without any NTP correction between subsequent runs of the metadata regenerate command.