Fix preloader mode in benchmarks

[s-matyukevich]

This PR fixes #6320

• It makes sure the benchmarks use preloaded message not only on the client but on the server as well if preloader=on option is provided.
• It adds support for preloader=on option in streaming mode (previously it was supported only in unconstrained mode) Initially I was going to add it to unary mode as well, but this would require using raw clientConnection instead of generated grpc stubs, so I decided to ask before doing that.

This PR ends up being not super useful for the use-case I had in mind for it. The problem I was trying to solve is that when testing #6309 I discovered that google.golang.org/protobuf/proto.MarshalOptions.marshal takes the wast majority of the inuse memory in the resulting memroy profile. I thought that by fixing preloader I'll be able to reduce the impact of this method on the overall memory usage and therefore make the impact of memory optimizations more visible. What ends up happening is that profiler results are misleading. Looks like marshal doesn't have significant impact on the overall memory usage - it is just a method that take relatively long time to execute, so memory allocated by it sometimes (but not always) ends up being counted in the inuse_memory profile, but all this memory is immediately deallocated after the method finishes. So enabling preloader does significantly reduce allocations per operation (which is expected because we are not calling marshal for every operation) but it doesn't have significant impact on the amount of memory reserved by the process in the operating system.

Still, I decided to submit this PR as you might find it useful for testing other use-cases, like this one #2432

RELEASE NOTES: N/A

[zasweq]

Can you please fix vet? I'll take a look at this next week :).

[zasweq]

Some minor nits. Looks solid overall though. I like everything going through helpers.

[easwars]

Seems mostly OK. Some minor comments.

[easwars]

Would it be possible to return an error from here instead of calling Fatal. Please see:
https://google.github.io/styleguide/go/best-practices#program-checks-and-panics and https://google.github.io/styleguide/go/decisions#dont-panic.

[s-matyukevich]

This seems to be a common pattern used in benchmarks logger.Fatal is used 6 times in the same file. If we want to propagate the error up to the main method and handle it there we should fix all the cases, I can do it in the same PR, but this would be an unrelated change.

[easwars]

Ok fair enough. If you have cycles, and would like to fix this in a follow-up PR, I would be happy to review it. Thanks.

[s-matyukevich]

@easwars @zasweq - I fixed all comments except for the one that is left unresolved. Let me know what I should do about it.

[zasweq]

LGTM. Thanks for the contribution.

[s-matyukevich]

I don't have the rights to merge it, can somebody please do it for me?

[easwars]

Thank you for your contribution!