src/histogram: Drop collect lock guard explicitly

Rusts drop semantics can be confusing sometimes. E.g. `let _ = l.lock()`
would drop the lock guard immediately whereas `let _guard = l.lock()`
would drop the guard in LIFO order at the end of the current scope.

Instead of relying on the above guarantee with `let _guard`, drop the
mutex guard explicitely hopefully making this less error prone in the
future.

Signed-off-by: Max Inden <mail@max-inden.de>

src/histogram.rs

@@ -393,7 +393,7 @@ impl HistogramCore {
     // remaining `observe` calls to finish on the previously hot now cold shard,
     // snapshot the data, update the now hot shard and reset the cold shard.
     pub fn proto(&self) -> proto::Histogram {
-        let _guard = self.collect_lock.lock().expect("Lock poisoned");
+        let _collect_guard = self.collect_lock.lock().expect("Lock poisoned");
 
         // `flip` needs to use AcqRel ordering to ensure the lock operation
         // above stays above and the histogram operations (especially the shard
@@ -450,6 +450,8 @@ impl HistogramCore {
         hot_shard.count.inc_by(overall_count);
         hot_shard.sum.inc_by(cold_shard_sum);
 
+        drop(_collect_guard);
+
         h
     }