dex: correct placement of arb metrics emission (#4535)

We noticed there were no metrics being emitted for the cost of the arb
search when no arb was found.

This fixes that, and should be backported into the next point release.

- [x] If this code contains consensus-breaking changes, I have added the
"consensus-breaking" label. Otherwise, I declare my belief that there
are not consensus-breaking changes, for the following reason:

  > Only changes metrics emission

crates/core/component/dex/src/component/arb.rs

@@ -63,6 +63,11 @@ pub trait Arbitrage: StateWrite + Sized {
             .checked_sub(&filled_input)
             .expect("filled input should always be <= flash loan amount");
 
+        // Record the duration of the arb execution now, since we've computed it
+        // and we might return early if it's zero-valued, but in that case we still
+        // want to record how long we spent looking for it.
+        metrics::histogram!(metrics::DEX_ARB_DURATION).record(arb_start.elapsed());
+
         // Because we're trading the arb token to itself, the total output is the
         // output from the route-and-fill, plus the unfilled input.
         let total_output = output + unfilled_input;
@@ -134,7 +139,6 @@ pub trait Arbitrage: StateWrite + Sized {
 
         // Emit an ABCI event detailing the arb execution.
         self_mut.record_proto(event::arb_execution(height, se));
-        metrics::histogram!(metrics::DEX_ARB_DURATION).record(arb_start.elapsed());
         return Ok(Value {
             amount: arb_profit,
             asset_id: arb_token,