metrics: HTTP status codes and inconsistent responses

[rvanasa]

Adds the following metrics:

• responses now records the HTTP status codes for JSON-RPC responses.
• inconsistent_responses counts the number of MultiRpcResult::Inconsistent(..) results for each combination of RPC method and hostname.

Another notable change (not represented in the diff of this PR) is that I adjusted the implementation of MultiCallResult::all_ok() in the forked ckETH codebase to return all results in the case of an error. Otherwise, an RPC service could bypass the agreement / consistency logic by returning an error for an otherwise successful RPC request.

[THLO]

an RPC service could bypass consensus

What do you mean by that? Every response to an HTTPS outcall goes through consensus, so there is no way to "bypass consensus".

[rvanasa]

What do you mean by that?

Oh right, sorry for the ambiguity; I was using "consensus" to refer to the agreement logic between RPC providers.

In ckETH, if two responses are successful while one returns an error, this is considered a "consistent error" (rather than three "inconsistent results"). We need to change this for EVM RPC because a compromised third-party RPC service could bypass the agreement logic by responding with an error for an otherwise successful request.