When using HLG you would not need metadata AFAIK (and it’s looking like HLG will be the defacto standard in broadcast)
HDR can work in 10bit, the HDR10 spec is based on that. although it can be extended to 12bit or more in HDR10+. The HLG wikipedia has some interesting info on SDR 8 bit vs SDR 10 bit vs HDR 10 bit.