Report an error if Timespans are used twice before are cleared

This allows catching multiple unintended start()/stop() call
cycles before a metric is cleared.

glean-core/src/metrics/timespan.rs

@@ -120,16 +120,33 @@ impl TimespanMetric {
             return;
         }
 
+        let mut report_value_exists: bool = false;
         glean.storage().record_with(&self.meta, |old_value| {
             if overwrite {
                 Metric::Timespan(elapsed, self.time_unit)
             } else {
                 match old_value {
-                    Some(old @ Metric::Timespan(..)) => old,
+                    Some(old @ Metric::Timespan(..)) => {
+                        // If some value already exists, report an error.
+                        // We do this out of the storage since recording an
+                        // error accesses the storage as well.
+                        report_value_exists = true;
+                        old
+                    }
                     _ => Metric::Timespan(elapsed, self.time_unit),
                 }
             }
         });
+
+        if report_value_exists {
+            record_error(
+                glean,
+                &self.meta,
+                ErrorType::InvalidValue,
+                "Timespan value already recorded. New value discarded.",
+                None,
+            );
+        };
     }
 
     /// **Test-only API (exported for FFI purposes).**

glean-core/tests/timespan.rs

@@ -114,6 +114,11 @@ fn second_timer_run_is_skipped() {
     metric.set_start(&glean, 0);
     metric.set_stop(&glean, duration);
 
+    // No error should be recorded here: we had no prior value stored.
+    assert!(
+        test_get_num_recorded_errors(&glean, metric.meta(), ErrorType::InvalidValue, None).is_err()
+    );
+
     let first_value = metric.test_get_value(&glean, "store1").unwrap();
     assert_eq!(duration, first_value);
 
@@ -122,6 +127,13 @@ fn second_timer_run_is_skipped() {
 
     let second_value = metric.test_get_value(&glean, "store1").unwrap();
     assert_eq!(second_value, first_value);
+
+    // Make sure that the error has been recorded: we had a stored value, the
+    // new measurement was dropped.
+    assert_eq!(
+        Ok(1),
+        test_get_num_recorded_errors(&glean, metric.meta(), ErrorType::InvalidValue, None)
+    );
 }
 
 #[test]