Transiting traceparent over custom transports

I'm implementing [grpc-node](https://github.com/grpc/grpc-node) support (specifically the `grpc` npm package) with the new [addPatch](https://www.elastic.co/guide/en/apm/agent/nodejs/current/agent-api.html#apm-add-patch) agent API.

I had guessed (incorrectly!) that if GRPC wasn't supported for `apm-agent-nodejs` that it wasn't supported for any Elastic APM agent. I've found, however, that at least `apm-agent-go` [has GRPC support](https://github.com/elastic/apm-agent-go/tree/master/module/apmgrpc), so I want to make sure that my implementation is fully compatible for distributed tracing.

I have three questions:

1) What is the officially supported way to obtain the correct APM context in `apm-agent-nodejs` to transit between services for distributed tracing?

I was looking at some of the existing code (link below) and am weary of using the `_context` property of `Span` found therein. Does the leading underscore mean this is not a public interface? If it is a private interface, is there an alternative way to obtain this information?

https://github.com/elastic/apm-agent-nodejs/blob/853c9ddde1ce2d671e3d478e103edeeac5f29539/lib/instrumentation/http-shared.js#L122-L130

2) Why are S3 hosts blacklisted in the `http-shared` module?

https://github.com/elastic/apm-agent-nodejs/blob/853c9ddde1ce2d671e3d478e103edeeac5f29539/lib/instrumentation/http-shared.js#L181-L183

3) When I transit this APM context from GRPC client to GRPC server, what is the correct way to do so?

Before I learned that GRPC was supported by some Elastic APM agents, I was just going to stuff the string representation of the context into some of the call's metadata, but looking more closely at `apm-agent-go` (disclaimer: I do not know golang) it looks like the GRPC implementation depends on the header name from the HTTP implementation:

https://github.com/elastic/apm-agent-go/blob/master/module/apmgrpc/server.go#L97-L98

Which means that when this value changes in a future version of the agent pending W3C standard freeze, the GRPC metadata key will also be changed.

https://github.com/elastic/apm-agent-go/blob/master/module/apmhttp/traceheaders.go#L31-L36

So, how can I make sure that my implementation for `node-grpc` is similarly responsive to this change?

Any other advice or pointers would be greatly appreciated!