Bump prost to `0.12`

[waynexia]

Is your feature request related to a problem? Please describe.

open-telemetry/opentelemetry-rust#1310

opentelemetry-rust dependents on both grpcio and tonic. On my application that dependents on opentelemetry-rust I have to bump tonic.

Describe the solution you'd like

Bump prost to 0.12

Describe alternatives you've considered
N/A

Additional context
N/A

[BusyJay]

Would you like to send a PR?