Its: Fix mismatch JSON and ProtoBuf message types

ProtoBuf is strongly typed. So all floating point metrics cannot be
"None". If they are, the import of metrics will fail, bringing down
the whole metrics with them.

Flag: EXEMPT bugfix
Test: Run relevant ITS tests and observe "None" floats are not written
Bug: 442698260
Change-Id: Ic5b3776941aea591333828524c80ae18d5a8e763