feat(pops): Challenger Metrics

[refcell]

Description

This PR adds metrics to the challenger service.

Challenger metrics is nearly identical to the proposer service, with the following additional endpoints. (More can be added in future prs, but these are the core data series we should collect to track the challenger service).

RecordValidOutput(l2ref eth.L2BlockRef)
RecordInvalidOutput(l2ref eth.L2BlockRef)
RecordOutputChallenged(l2ref eth.L2BlockRef)

Background

Building off #5568, this PR introduces new changes to the op-challenger service.

The op-challenger is a naive, honest challenge agent that can be run as a standalone service for permissionless output proposal validation.

In the initial iteration of PoPs, only a permissioned set of challenge agents can dispute outputs as part of the Attestation Dispute Game. But once Fault Dispute Games are implemented, disputing outputs will be permissionless.

Metadata

Fixes CLI-3943

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Semgrep found 1 no-fprintf-to-responsewriter finding:

• proxyd/tools/mockserver/handler/handler.go: L76

Detected 'Fprintf' or similar writing to 'http.ResponseWriter'. This bypasses HTML escaping that prevents cross-site scripting vulnerabilities. Instead, use the 'html/template' package to render data to users.

_{Created by no-fprintf-to-responsewriter.}

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