Fix: Address unrelated clippy warning (double_ended_iterator_last)

Use `.next_back()` instead of `.last()` on a `DoubleEndedIterator` in `tests/metrics_publishing.rs` to satisfy the `clippy::double_ended_iterator_last` lint, which was likely surfaced by CI flags (`-D warnings`) or a toolchain update.

Author: alessandrobologna

tests/metrics_publishing.rs

@@ -625,7 +625,7 @@ where
                                 .data_points
                                 .iter()
                                 .map(|data_point| data_point.value)
-                                .last()
+                                .next_back()
                                 .unwrap()
                         );