op-reth!: use standard RPC metrics for historical proofs

[joshklop]

Stacked on #23046. The historical-proofs override currently duplicates Reth's built-in per-method RPC instrumentation, so this switches operators to the standard reth_rpc_server_calls_*{method="eth_getProof"} series and documents the migration and its accounting differences.

This intentionally accepts that Reth exposes only volume for batch entries and does not cover authenticated RPC calls or standalone notifications. The in-tree Kona hosts, Go clients, and proof benchmark all issue individual eth_getProof calls rather than JSON-RPC batches.

A seven-day comparison of successful-call counter increases found an exact match on production proof nodes and differences of only -0.0014% to -0.0078% on high-volume I nodes. This query reports the percentage difference per network and namespace, where zero means the counters are equivalent:

(
  100 * (
    (
      sum by (stack_optimism_io_network, namespace) (
        increase(reth_rpc_server_calls_successful_total{method="eth_getProof"}[$__range])
      )
      /
      sum by (stack_optimism_io_network, namespace) (
        increase(reth_optimism_rpc_eth_api_ext_get_proof_successful_responses[$__range])
      )
    ) - 1
  )
)
and on (stack_optimism_io_network, namespace)
(
  sum by (stack_optimism_io_network, namespace) (
    increase(reth_optimism_rpc_eth_api_ext_get_proof_successful_responses[$__range])
  ) > 100
)

Known rollout dependency: the Grafana dashboard op-reth: NVMe vs hyperdisk has five queries across its proof-serving panels that use the removed metrics. Those queries must be migrated before this change is deployed.

No additional integration test is included because the metric middleware and batch accounting are owned and tested upstream; the local change only removes handler-level instrumentation, while the operator documentation captures the pinned upstream semantics.

BREAKING CHANGE: Historical-proofs operators must migrate dashboards and alerts from reth_optimism_rpc_eth_api_ext_get_proof_* to reth_rpc_server_calls_* filtered by method="eth_getProof". The public EthApiExtMetrics Rust type is removed. Per-entry batch outcomes and latency, authenticated RPC calls, and standalone notification calls are no longer measured.

[joshklop]

Closing in favour of tracking this as ethereum-optimism/protocol-team#455, to keep it out of the reth v2.6.0 stack. The branch is kept for reference.