otelgrpc: performance issue about otlpreceiver invoke proto.Size slow path

[hanjm]

The opentelemetry collector otlpreceiver consume lots of cpu for proto.Size, marshal to buffer, get buffer size.

related open-telemetry/opentelemetry-collector#6947

I think it shoud try get size from asserting interface Size() int, then try to use proto.Size

[bogdandrutu]

A possible better option is to actually implement stats.Handler (which was designed for this) instead of unary/streaming interceptors. The stats handlers have all the lengths pre-calculated and available.

See OpenCensus https://github.com/census-instrumentation/opencensus-go/blob/v0.24.0/plugin/ocgrpc/server.go#LL32C4-L32C4

Another advantage is that it works with any encoding not only protobuf and gRPC supports other encodings.

[Aneurysm9]

I have created #3168 to remove the attribute for the time being. I agree that the stats.Handler interface looks like it is the correct way to go in the longer term, but I do not know when such an implementation can be created.