  Space coronagraph Metis on board of the Solar Orbiter offers us new
capabilities for studying eruptive prominences and coronal mass ejections
(CME). Its two spectral channels, hydrogen L$\alpha$ and visible-light (VL)
will provide, for the first time, co-aligned and co-temporal images to study
dynamics and plasma properties of CMEs. Moreover, with the VL channel (580 -
640 nm) we find an exciting possibility to detect the helium D$_3$ line (587.73
nm) and its linear polarization. The aim of this study is to predict the
diagnostics potential of this line regarding the CME thermal and magnetic
structure. For a grid of models we first compute the intensity of the D$_3$
line together with VL continuum intensity due to Thomson scattering on core
electrons. We show that the Metis VL channel will detect a mixture of both,
with predominance of the helium emission at intermediate temperatures between
30 - 50,000 K. Then we use the code HAZEL to compute the degree of linear
polarization detectable in the VL channel. This is a mixture of D$_3$
scattering polarization and continuum polarization. The former one is lowered
in the presence of a magnetic field and the polarization axis is rotated (Hanle
effect). Metis has the capability of measuring $Q/I$ and $U/I$ polarization
degrees and we show their dependence on temperature and magnetic field. At
$T$=30,000 K we find a significant lowering of $Q/I$ which is due to strongly
enhanced D$_3$ line emission, while depolarization at 10 G amounts roughly to
10 \%.
