Remove the custom metrics protobuf now that we think no one is using it

[kentquirk]

It looks like the last team still using the old int-style metrics from v0.7 of OTLP finally stopped.

I think this means that we could, at long last, remove our unholy custom protobuf for metrics ingest.

There are probably more queries to do that we can use to make sure that there aren't other AWS-related things still dependent on it.

[MikeGoldsmith]

We should also update our docs to say we only accept OTLP v1.0+ (int-based metrics were removed in v0.17).

Note: we won't officially be off the custom OTLP until shepherd is updated with a new release of Husky.

[TylerHelmuth]

We learned that customers are still using the deprecated labels field from old OTLP

[TylerHelmuth]

https://honeycomb.quip.com/oE5HAS68Tpwj/OTLP-proto-deprecation-guidance