[metrics] Add gas fee telemetry series to charon-metrics exporter

[obchain]

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

Scope

The Grafana dashboard (deploy/grafana/charon.json) omits a gas-fee panel because backing series do not exist. Add:

• charon_gas_base_fee_wei (gauge) — latest observed EIP-1559 baseFee per block, labeled by chain
• charon_gas_priority_fee_wei (gauge) — priority fee bid used for the last submission attempt
• charon_gas_max_fee_wei (gauge) — maxFeePerGas on the last submission
• charon_gas_ceiling_skips_total (counter) — opportunities dropped because gas cost exceeded the configured ceiling, labeled by reason

Emission site: crates/charon-executor/src/gas.rs (GasOracle::decide). Already returns GasDecision; wire metric updates inline.

Acceptance

• New series registered in charon-metrics
• GasOracle::decide updates series on every call
• Dashboard panel added post-merge referencing these series

References

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

[obchain]

Fixed on feat/22-prometheus-metrics (7258cc7): added charon_gas_base_fee_wei, charon_gas_priority_fee_wei, charon_gas_max_fee_wei (gauges, label: chain) and charon_gas_ceiling_skips_total (counter, labels: chain, reason). Emitted from GasOracle::fetch_params; max_fee gauge deliberately left untouched on the ceiling-skip branch so its semantic stays 'last submitted tx fee'. GasOracle::new now takes a chain argument.