configurable middleware and metrics & http middleware

[smatthewenglish]

note: this compiles, but i don't think it would achieve the goal of allowing a builder to pass arbitrary middleware, as you can see here

another thing i tried was splitting the building of the server and the starting of the server into two distinct steps, but the trait bounds started to get very complicated and ultimately i wasn't able to get it working

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this trait bound will get the server to start correctly

RpcMiddleware: Layer<RpcRequestMetricsService<RpcService>> + Send + 'static,

but i wish that it was more dynamic, i tried something like this

RpcMiddleware: Layer<Box<dyn RpcServiceT<RpcService>>> + Send + 'static,

but that didnt work.

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i adjusted the visibility of RpcRequestMetricsService to avoid this error

type `RpcRequestMetricsService<RpcService>` is more private than the item `RpcServerConfig::<HttpMiddleware, RpcMiddleware>::start`
`#[warn(private_bounds)]` on by default

'cause it seems like a better solution than using #[warn(private_bounds)]

[mattsse]

cool, but can we do this in two steps, if I read this correctly, then this does two things

will investigate the trait bounds

[mattsse]

could we first install the rpcmiddleware before we look at the HTTP middleware?

[smatthewenglish]

yes, sounds good. i just made that change