docs: add comment

Author: josecelano

packages/metrics/src/sample_collection.rs

@@ -15,6 +15,11 @@ pub struct SampleCollection<T> {
 }
 
 impl<T> SampleCollection<T> {
+    // IMPORTANT: It should never allow mutation of the samples because it would
+    // break the invariants. If the sample's `LabelSet` is changed, it can
+    // create duplicate `LabelSet`s even if the `LabelSet` in the `HashMap` key
+    // is unique.
+
     /// # Panics
     ///
     /// Panics if there are duplicate `LabelSets` in the provided samples.
@@ -43,7 +48,7 @@ impl SampleCollection<Counter> {
         let sample = self
             .samples
             .entry(labels.clone())
-            .or_insert_with(|| Sample::new(Counter::new(0), time, labels.clone()));
+            .or_insert_with(|| Sample::new(Counter::default(), time, labels.clone()));
 
         sample.increment(time);
     }
@@ -54,7 +59,7 @@ impl SampleCollection<Gauge> {
         let sample = self
             .samples
             .entry(labels.clone())
-            .or_insert_with(|| Sample::new(Gauge::new(0.0), time, labels.clone()));
+            .or_insert_with(|| Sample::new(Gauge::default(), time, labels.clone()));
 
         sample.set(value, time);
     }