Add multi-threaded test for Counter (open-telemetry#1845)

Co-authored-by: Cijo Thomas <cijo.thomas@gmail.com>

opentelemetry-sdk/src/metrics/mod.rs

@@ -150,6 +150,7 @@ mod tests {
     use rand::{rngs, Rng, SeedableRng};
     use std::borrow::Cow;
     use std::sync::{Arc, Mutex};
+    use std::thread;
 
     // Run all tests in this mod
     // cargo test metrics::tests --features=metrics,testing
@@ -1032,6 +1033,61 @@ mod tests {
         assert!(resource_metrics.is_empty(), "No metrics should be exported as no new measurements were recorded since last collect.");
     }
 
+    #[tokio::test(flavor = "multi_thread", worker_threads = 1)]
+    async fn counter_multithreaded() {
+        // Run this test with stdout enabled to see output.
+        // cargo test counter_multithreaded --features=metrics,testing -- --nocapture
+
+        counter_multithreaded_aggregation_helper(Temporality::Delta);
+        counter_multithreaded_aggregation_helper(Temporality::Cumulative);
+    }
+
+    fn counter_multithreaded_aggregation_helper(temporality: Temporality) {
+        // Arrange
+        let mut test_context = TestContext::new(temporality);
+        let counter = Arc::new(test_context.u64_counter("test", "my_counter", None));
+
+        let mut update_threads = vec![];
+        for _ in 0..10 {
+            let counter = Arc::clone(&counter);
+
+            update_threads.push(thread::spawn(move || {
+                counter.add(1, &[KeyValue::new("key1", "value1")]);
+                counter.add(1, &[KeyValue::new("key1", "value1")]);
+                counter.add(1, &[KeyValue::new("key1", "value1")]);
+                counter.add(1, &[KeyValue::new("key1", "value1")]);
+                counter.add(1, &[KeyValue::new("key1", "value1")]);
+            }));
+        }
+
+        for thread in update_threads {
+            thread.join().unwrap();
+        }
+
+        test_context.flush_metrics();
+
+        // Assert
+        let sum = test_context.get_aggregation::<data::Sum<u64>>("my_counter", None);
+        // Expecting 2 time-series.
+        assert_eq!(sum.data_points.len(), 1);
+        assert!(sum.is_monotonic, "Counter should produce monotonic.");
+        assert_eq!(sum.temporality, temporality);
+        if let Temporality::Cumulative = temporality {
+            assert_eq!(
+                sum.temporality,
+                Temporality::Cumulative,
+                "Should produce cumulative"
+            );
+        } else {
+            assert_eq!(sum.temporality, Temporality::Delta, "Should produce delta");
+        }
+
+        // find and validate key1=value2 datapoint
+        let data_point1 = find_datapoint_with_key_value(&sum.data_points, "key1", "value1")
+            .expect("datapoint with key1=value1 expected");
+        assert_eq!(data_point1.value, 50); // Each of the 10 update threads record measurements summing up to 5.
+    }
+
     fn histogram_aggregation_helper(temporality: Temporality) {
         // Arrange
         let mut test_context = TestContext::new(temporality);