with line as (select ST_GeomFromText('LINESTRINGM(0 0 10, 2 4 100, 5 5 10)') as geom)
    select avg(ST_M(ST_PointN(geom, num))) as m
    from line,
(select generate_series(1, (select ST_NumPoints(geom) from line)) as num)
    as series;