Deprecate parse_descriptor_proto()

[randomdross]

As per: #461 (comment)
"This is part of the generated protobuf code. It'd be reasonable to regen this and see if the unsafe bits have disappeared. It'd also be willing to open up an issue for deprecation in the next release, removal in the release after."

[blt]

This is a little off the mark. We can't excise a small bit of a piece of generated code. The options are:

• attempt to regen the code and see if the unsafe block has magically disappeared,
• merge changes upstream to adjust how code is generated or
• deprecate the whole protobuf/prometheus reporting subsystem.

[blt]

@randomdross I pulled off my assignment. I'm happy to review patches for this, discuss a deprecation glide path, that said.

[randomdross]

No issues w/that, I just wanted to make sure it doesn't fall off the radar.