refactor(nns): Follow up on PR 3090 (dfinity#3773)

dfinity#3090

A couple of issues:

1. Moved comment.
2. Use < instead of checked_sub to detect when right operand is greater
than left.

Author: daniel-wong-dfinity-org

rs/nns/governance/src/neuron_store/metrics.rs

@@ -111,19 +111,23 @@ impl NeuronMetrics {
         now_seconds: u64,
         neuron: &Neuron,
     ) {
+        // The substraction here assumes that the neuron was not refreshed in
+        // the future. (This doesn't always hold in tests though, due to the
+        // difficulty of constructing realistic data/scenarios.)
         let seconds_since_voting_power_refreshed =
-            // Here, we assume that the neuron was not refreshed in the future.
-            // This doesn't always hold in tests though, due to the difficulty
-            // of constructing realistic data/scenarios.
             now_seconds.saturating_sub(neuron.voting_power_refreshed_timestamp_seconds());
-        let Some(seconds_losing_voting_power) = seconds_since_voting_power_refreshed
-            .checked_sub(voting_power_economics.get_start_reducing_voting_power_after_seconds())
-        else {
+
+        let is_recently_refreshed = seconds_since_voting_power_refreshed
+            < voting_power_economics.get_start_reducing_voting_power_after_seconds();
+        if is_recently_refreshed {
             return;
-        };
+        }
 
-        if seconds_losing_voting_power < voting_power_economics.get_clear_following_after_seconds()
-        {
+        let is_moderately_refreshed = seconds_since_voting_power_refreshed
+            < voting_power_economics
+                .get_start_reducing_voting_power_after_seconds()
+                .saturating_add(voting_power_economics.get_clear_following_after_seconds());
+        if is_moderately_refreshed {
             self.declining_voting_power_neuron_subset_metrics.increment(
                 voting_power_economics,
                 now_seconds,