gas-oracle: fix the gas price metric

The `gas-price` metric was renamed to `gas_price`
as using a `-` is invalid syntax in a prometheus metric.

Fixes https://github.com/ethereum-optimism/optimism/issues/1819

.changeset/flat-bags-chew.md

+---
+'@eth-optimism/gas-oracle': patch
+---
+
+Fix the gas oracle gas price prometheus metric

go/gas-oracle/oracle/updater_interface.go

@@ -19,7 +19,7 @@ import (
 var (
 	txSendCounter           = metrics.NewRegisteredCounter("tx/send", ometrics.DefaultRegistry)
 	txNotSignificantCounter = metrics.NewRegisteredCounter("tx/not-significant", ometrics.DefaultRegistry)
-	gasPriceGauge           = metrics.NewRegisteredGauge("gas-price", ometrics.DefaultRegistry)
+	gasPriceGauge           = metrics.NewRegisteredGauge("gas_price", ometrics.DefaultRegistry)
 	txConfTimer             = metrics.NewRegisteredTimer("tx/confirmed", ometrics.DefaultRegistry)
 	txSendTimer             = metrics.NewRegisteredTimer("tx/send", ometrics.DefaultRegistry)
 )