Allow protobuf to be excluded with push feature

[Hoverbear]

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

In #278 we talked about allowing protobuf to be excluded when push is enabled.

Fulfilling this issue would mean one could use the push feature without using protobuf.

[xjewer]

if anyone interesting in it, there is a PR in my fork https://github.com/xjewer/rust-prometheus/pull/1