Fix Family::get_or_create

The method should not overwrite an existing key after it obtains the write lock.

Consider the following scenario with `Family<LabelSet, Gauge>` used by threads A and B:

1. A and B both call `family.get_or_insert(&label_set)`
2. A and B both acquire read lock and finds no key
3. A gets write lock and inserts a new gauge with value 0
4. A increments returned gauge to 1
5. B gets write lock and inserts a new gauge with value 0
6. B increments returned gauge to 1
7. A decrements gauge back to 0
8. B decrements gauge which now overflows to `u64::MAX`

Signed-off-by: Anthony Ramine <nox@nox.paris>

src/metrics/family.rs

@@ -221,10 +221,10 @@ impl<S: Clone + std::hash::Hash + Eq, M, C: MetricConstructor<M>> Family<S, M, C
             return metric;
         }
 
-        let mut write_guard = self.metrics.write();
-        write_guard.insert(label_set.clone(), self.constructor.new_metric());
-
-        drop(write_guard);
+        self.metrics
+            .write()
+            .entry(label_set.clone())
+            .or_insert_with(|| self.constructor.new_metric());
 
         RwLockReadGuard::map(self.metrics.read(), |metrics| {
             metrics