Bring the metrics-observer protobufs up to date

[ceejbot]

Aligned the proto file in the observer sub-crate with the one in the TCP exporter. Updated the observer message handling code to use the new structures.

This format de-sync will occur again if the writer code is changed without also updating the observer. The right solution here is to have the two crates share the protobuf file, but that requires more structural tinkering than a newcomer like me should do :).

Satisfied the clippy lints complaining about type complexity by creeating some subtypes.

I note that in order to test with rustc 1.56.1, I had to pin the version of mockall used by metrics-util to 0.11.0.

This fixes #325 "metrics-observer is not available".

[tobz]

I've left a few comments about specific changes, but generally, I'd like to see qualified references (i.e. match proto::event::Event::decode { ...) be changed to drop the qualified portion (proto::event) and move those types up to the imports at the top of the file.

This keeps things cleaner in the business logic and is the prevailing style throughout the project.

Generally speaking, though, these changes make sense and it'll be good to get the TCP exporter back on track.

[tobz]

Released as metrics-exporter-tcp@v0.8.0 and metrics-observer@v0.2.0.

Thanks again for your contribution!