Document that gRPC's how to use well know types (google.protobuf.*)

[olegbespalov]

What?

In k6 v0.47 we fixed the unmarshaling of the well-known GRPC types (google.protobuf.*)

But their usage needs to be clarified for our customers.

E.g.:

• Support Value well known type xk6-grpc#55
• https://community.grafana.com/t/k6-is-sending-empty-text-for-datetime-string/105656

We need to provide more examples and document them.

Why?

It needs to be clarified for our users.

[mstoykov]

Just noting that in particular they are marshalled and unmarshalled to their JSON representation as I found out in grafana/k6#3426