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 to the project
should do :).

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

The prost generated message wrapper type is named `Event`, and this
overlaps with our metrics Event type that is in its payload. Named
the wrapper `ProstMessage` to make its meaning more clear. This one
is only used once and might reasonably be used fully-qualified inline.

Also, fixed the typo in MetadataValue.