Fix metric export data for unused gauges (#1363)

Author: jtescher

opentelemetry-sdk/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## vNext
 
+### Fixed
+
+- Fix metric export corruption if gauges have not received a last value. (#1363)
+
 ## v0.21.0
 
 ### Added

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

@@ -55,8 +55,12 @@ impl<T: Number<T>> LastValue<T> {
     pub(crate) fn compute_aggregation(&self, dest: &mut Vec<DataPoint<T>>) {
         let mut values = match self.values.lock() {
             Ok(guard) if !guard.is_empty() => guard,
-            _ => return,
+            _ => {
+                dest.clear(); // poisoned or no values recorded yet
+                return;
+            }
         };
+
         let n = values.len();
         if n > dest.capacity() {
             dest.reserve(n - dest.capacity());