Support persistent session in IntrospectToken method of GRPC client, PLTFRM-72444

[Semerokozlyat]

Support persistent session in IntrospectToken method of GRPC client.

Partly PLTFRM-72444

[MikeYast]

It looks better to use the metadata.MD wrapper here in case of multiple calls to ValueFromIncomingContext(), otherwise, we have redundant deserialization work.

[Semerokozlyat]

Reworked, but this code is a part of Token Introspector mock and amount of allocation is not so critical here.

[MikeYast]

It's better to use the metadata.MD wrapper here when making multiple calls to AppendToOutgoingContext() or ValueFromIncomingContext(), as otherwise, redundant (de)serialization work occurs. Additionally, with AppendToOutgoingContext(), you will have +N redundant memory allocations, where N is the number of AppendToOutgoingContext() calls.

[Semerokozlyat]

No, I cannot agree with this proposal:

1. AppendToOutgoingContext method is called here either one or (with requestIDProvider) two times - not event tens or hundreds of times, optimization is not obvious.
2. AppendToOutgoingContext does additional data validation, keys lowercase conversion and so on - with "NewIncomingContext(ctx context.Context, md MD)" method we must do these manually which is easy to forget.

[MikeYast]

Using more than one call to AppendToOutgoingContext() is inefficient—it causes redundant memory allocations and unnecessary CPU usage. If you need to set several context keys, consider doing something like this:
md := make(metadata.MD)
md.Append("key1", "value1")
md.Append("key2", "value2")
ctx = metadata.NewOutgoingContext(ctx, md)