otelgrpc: Add status code attribute in accordance with the spec

[matej-g]

Although span status is no longer aligned with gRPC codes, according to the recent spec update, gRPC code should be reflected in the attributes.

Relevant part of the spec: https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/semantic_conventions/rpc.md#grpc-attributes

The gRPC interceptor should be adjusted accordingly.

[matej-g]

@MrAlias feel free to assign this to me as well