FEATURE: Export metric for highest sequence usage

[nbianca]

Plugin already exports postgres_highest_sequence, but the value cannot be reliable used for alerts because it is not linked with the maximum value of the column that uses it.

In other words, a sequence may be used with an integer or a bigint column, which have very different maximum values. The highest last_value reported may be cause of concern for an integer column because it is close to the limit, but not a problem for a bigint column.