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Simplify GraphQL subscription load balancing

Matthias Käppler requested to merge 402999-simplify-graphql-lb into master

What does this MR do and why?

Simplifies GraphQL subscription load balancing. This is a refactor and not user facing.

We introduced database load balancing for GraphQL subs in !117678 (merged). In that MR we hooked into the GraphQL-ruby serializer to inject custom payload through Redis PubSub so readers know how far they are behind the writer.

I found a better way to wrap event payloads that does not involve overriding the internal serializer, but simply wraps the original object in a Hash. The bit I missed before was that graphql-ruby is still able to resolve entities recursively this way. This also prepares this class for future extensions that will require a custom implementation of execute_update and execute_all anyway, such as instrumenting query executions, which we are looking to do in #402717.

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Related to #402999 (closed)

Edited by Matthias Käppler

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