A tibble object with 1 row for each subject, a column for subject id and
a column for HBGI values is returned. NA glucose values are
omitted from the calculation of the HBGI.
HBGI is calculated by \(1/n * \sum (10 * fg_i ^2)\),
where \(fg_i = max(0, 1.509 * (log(G_i)^{1.084} - 5.381)\),
G_i is the ith Glucose measurement for a subject, and
n is the total number of measurements for that subject.