v2.1.0

Changes:

• New metrics were added: ntp_rtt_seconds, ntp_reference_timestamp_seconds, ntp_root_delay_seconds, ntp_root_dispersion_seconds, ntp_root_distance_seconds, ntp_precision_seconds, ntp_leap
• The ntp_stratum metric now has the label server, and is reported separately for each server.
• Go got updated to version 1.18.