Fix collection of gas price metric (#2366)

## Description
This PR fixes the collection of `importer_gas_price_for_block` metric.

### Before requesting review
- [X] I have reviewed the code myself

CHANGELOG.md

@@ -6,6 +6,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 
+### Fixed
+- [2366](https://github.com/FuelLabs/fuel-core/pull/2366): The `importer_gas_price_for_block` metric is properly collected.
+
 ### Added
 - [2321](https://github.com/FuelLabs/fuel-core/pull/2321): New metrics for the txpool: "The size of transactions in the txpool" (`txpool_tx_size`), "The time spent by a transaction in the txpool in seconds" (`txpool_tx_time_in_txpool_seconds`), The number of transactions in the txpool (`txpool_number_of_transactions`), "The number of transactions pending verification before entering the txpool" (`txpool_number_of_transactions_pending_verification`), "The number of executable transactions in the txpool" (`txpool_number_of_executable_transactions`), "The time it took to select transactions for inclusion in a block in nanoseconds" (`txpool_select_transaction_time_nanoseconds`), The time it took to insert a transaction in the txpool in milliseconds (`txpool_insert_transaction_time_milliseconds`).
 

crates/metrics/src/importer.rs

@@ -74,7 +74,7 @@ impl Default for ImporterMetrics {
         registry.register(
             "importer_gas_price_for_block",
             "The gas prices used in a block",
-            transactions_per_block.clone(),
+            gas_price.clone(),
         );
 
         Self {