Fix metric for total block production time (sigp#4794)

## Proposed Changes

Fix the misplacement of the total block production time metric, which occurred during a previous refactor.

Total block production times are no longer skewed low (data from Holesky + blockdreamer):

```
# HELP beacon_block_production_seconds Full runtime of block production
# TYPE beacon_block_production_seconds histogram
beacon_block_production_seconds_bucket{le="0.005"} 0
beacon_block_production_seconds_bucket{le="0.01"} 0
beacon_block_production_seconds_bucket{le="0.025"} 0
beacon_block_production_seconds_bucket{le="0.05"} 0
beacon_block_production_seconds_bucket{le="0.1"} 0
beacon_block_production_seconds_bucket{le="0.25"} 0
beacon_block_production_seconds_bucket{le="0.5"} 37
beacon_block_production_seconds_bucket{le="1"} 65
beacon_block_production_seconds_bucket{le="2.5"} 66
beacon_block_production_seconds_bucket{le="5"} 66
beacon_block_production_seconds_bucket{le="10"} 66
beacon_block_production_seconds_bucket{le="+Inf"} 66
beacon_block_production_seconds_sum 34.225780452
beacon_block_production_seconds_count 66
```

## Additional Info

Cheers to @jimmygchen for helping spot this.

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -3978,6 +3978,9 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
         validator_graffiti: Option<Graffiti>,
         verification: ProduceBlockVerification,
     ) -> Result<BeaconBlockAndState<T::EthSpec, Payload>, BlockProductionError> {
+        metrics::inc_counter(&metrics::BLOCK_PRODUCTION_REQUESTS);
+        let _complete_timer = metrics::start_timer(&metrics::BLOCK_PRODUCTION_TIMES);
+
         // Part 1/2 (blocking)
         //
         // Load the parent state from disk.
@@ -4012,9 +4015,6 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
         self: &Arc<Self>,
         slot: Slot,
     ) -> Result<(BeaconState<T::EthSpec>, Option<Hash256>), BlockProductionError> {
-        metrics::inc_counter(&metrics::BLOCK_PRODUCTION_REQUESTS);
-        let _complete_timer = metrics::start_timer(&metrics::BLOCK_PRODUCTION_TIMES);
-
         let fork_choice_timer = metrics::start_timer(&metrics::BLOCK_PRODUCTION_FORK_CHOICE_TIMES);
         self.wait_for_fork_choice_before_block_production(slot)?;
         drop(fork_choice_timer);