[metrics] Add RPC-latency series to charon-metrics exporter

[obchain]

Tracking issue filed from PR #54 dashboard review (#283).

Scope

The Grafana dashboard (deploy/grafana/charon.json) omits an RPC-latency panel because backing series do not exist. Add:

• charon_rpc_call_duration_seconds (histogram) — labeled by method (eth_call, eth_getBlockByNumber, eth_sendRawTransaction, eth_getLogs) and endpoint_kind (public, private)
• charon_rpc_errors_total (counter) — labeled by method and error_kind (timeout, rejected, connection_lost)
• charon_rpc_connection_reconnects_total (counter) — labeled by endpoint_kind

Emission sites: wherever an alloy Provider call crosses the network — scanner health reads, executor submit, simulation calls. A middleware or wrapper around the Provider is the cleanest integration point.

Acceptance

• Histograms with sensible default buckets (1ms..30s)
• All public RPC entry points emit duration + error classification
• Dashboard panel added post-merge

References

• PR feat(grafana): importable dashboard JSON for Charon metrics #54 omission list

[obchain]

Fixed on feat/22-prometheus-metrics (7258cc7): added charon_rpc_call_duration_seconds histogram (labels: method, endpoint_kind; buckets 1ms..30s, registered via set_buckets_for_metric/Matcher::Full), charon_rpc_errors_total (labels: method, error_kind in {timeout, rejected, connection_lost}), and charon_rpc_connection_reconnects_total (label: endpoint_kind). Ergonomic helper time_rpc<F,T>(method, endpoint_kind, fut) wraps any future and records into the histogram on both branches — adopted at the eth_call simulation gate, the eth_sendRawTransaction submit path, and the pending-tx lookup in the mempool monitor. Block listener and mempool monitor bump the reconnect counter on every reconnect attempt. Histogram-bucket rationale documented in rustdoc alongside the bucket const. An alloy middleware layer was considered but alloy 0.8's Provider trait does not expose a stable method-name hook; a per-call-site wrapper keeps the method label verbatim.