Add separate buffer used in collect_and_reset

opentelemetry-sdk/src/metrics/internal/mod.rs

@@ -8,7 +8,7 @@ mod sum;
 use core::fmt;
 use std::collections::hash_map::Entry;
 use std::collections::HashMap;
-use std::mem::replace;
+use std::mem::swap;
 use std::ops::{Add, AddAssign, DerefMut, Sub};
 use std::sync::atomic::{AtomicBool, AtomicI64, AtomicU64, Ordering};
 use std::sync::{Arc, Mutex, RwLock};
@@ -64,6 +64,8 @@ where
     sorted_attribs: Mutex<HashMap<Vec<KeyValue>, Arc<A>>>,
     /// Configuration for an Aggregator
     config: A::InitConfig,
+    /// Swap with `sorted_attribs` on every `collect_and_reset`.
+    for_collect_after_reset: Mutex<HashMap<Vec<KeyValue>, Arc<A>>>,
 }
 
 impl<A> ValueMap<A>
@@ -76,9 +78,10 @@ where
                 tracker: A::create(&config),
                 is_set: AtomicBool::new(false),
             },
-            all_attribs: RwLock::new(HashMap::new()),
-            sorted_attribs: Mutex::new(HashMap::new()),
+            all_attribs: RwLock::new(Default::default()),
+            sorted_attribs: Mutex::new(Default::default()),
             config,
+            for_collect_after_reset: Mutex::new(Default::default()),
         }
     }
 
@@ -168,11 +171,14 @@ where
     where
         MapFn: FnMut(Vec<KeyValue>, A) -> Res,
     {
+        let mut to_collect = self
+            .for_collect_after_reset
+            .lock()
+            .unwrap_or_else(|err| err.into_inner());
         // reset sorted trackers so new attributes set will be written into new hashmap
-        let trackers = match self.sorted_attribs.lock() {
+        match self.sorted_attribs.lock() {
             Ok(mut trackers) => {
-                let new = HashMap::with_capacity(trackers.len());
-                replace(trackers.deref_mut(), new)
+                swap(trackers.deref_mut(), to_collect.deref_mut());
             }
             Err(_) => return,
         };
@@ -182,7 +188,7 @@ where
             Err(_) => return,
         };
 
-        prepare_data(dest, trackers.len());
+        prepare_data(dest, to_collect.len());
 
         if self.no_attribs.is_set.swap(false, Ordering::AcqRel) {
             dest.push(map_fn(
@@ -191,7 +197,7 @@ where
             ));
         }
 
-        for (attrs, tracker) in trackers.into_iter() {
+        for (attrs, tracker) in to_collect.drain() {
             let tracker = Arc::into_inner(tracker).expect("the only instance");
             dest.push(map_fn(attrs, tracker));
         }