My rule of thumb is to always buy at least one, if not two, spare batteries any time I buy a battery dependent camera. This policy has done me well over the years.
Leica USA responded to my query in January when this issue of spares for the M10 series came up: they were able to sell me a second spare, so I now have four batteries and two M10 bodies (M10-M, M10-R). If my past experience is any measure, these two cameras and four batteries will out last me ...
I'm sure this battery shortage is a relatively short term thing driven by global market issues outside of Leica's control. It's much less serious than the situation in 2005 when Polaroid stopped making film ... rendering all my SX-70 cameras potentially just shelf ornaments forever. Happily, the future smiled on us, The Impossible Project re-invented the film, and Polaroid is now healthy enough to continue in operation for the foreseeable future.
There's always hope.
G