fix(promql): report 0 from ts_of_last_over_time for negative timestamps

Prometheus 3.15 takes the max of the last float timestamp and the
histogram one, which defaults to 0 without histograms.

Signed-off-by: Dennis Zhuang <killme2008@gmail.com>

src/promql/src/range_eval/function.rs

@@ -125,10 +125,11 @@ impl RangeFunction {
             Self::TsOfFirstOverTime => {
                 Box::new(move |window| Some(seconds(window.timestamps[0] - offset_ms)))
             }
+            // Prometheus takes the max with the histogram timestamp, 0 when there is none, so a
+            // negative timestamp reports 0.
             Self::TsOfLastOverTime => Box::new(move |window| {
-                Some(seconds(
-                    window.timestamps[window.timestamps.len() - 1] - offset_ms,
-                ))
+                let last = window.timestamps[window.timestamps.len() - 1] - offset_ms;
+                Some(seconds(last.max(0)))
             }),
             Self::TsOfMinOverTime => Box::new(move |window| {
                 Some(seconds(timestamp_of_extremum(&window, false) - offset_ms))

src/promql/src/range_eval/tests.rs

@@ -1026,3 +1026,29 @@ fn waiting_for_a_point_drops_samples_behind_its_window() {
     let last = chunk(&[(100_000, vec![Some(1.0)])], 1, 0, true);
     assert!(evaluator.push(&last).unwrap().is_some());
 }
+
+/// Prometheus 3.15 reports `ts_of_last_over_time` as the max of the last float timestamp and
+/// a histogram timestamp that defaults to 0, so a window of negative timestamps reports 0;
+/// `ts_of_first_over_time` takes the min and keeps them.
+#[test]
+fn ts_of_last_over_time_reports_zero_for_negative_timestamps() {
+    let window = RangeWindow {
+        range_ms: 2_000,
+        offset_ms: 0,
+        anchor_ms: None,
+    };
+    let rows: Vec<Row> = vec![(-1_000, vec![Some(7.0)])];
+    for (function, expected) in [
+        (RangeFunction::TsOfLastOverTime, 0.0),
+        (RangeFunction::TsOfFirstOverTime, -1.0),
+    ] {
+        let mut evaluator =
+            RangeEvaluator::try_new(&raw_schema(1), grid(0, 1_000, 1), window, function).unwrap();
+        let series = evaluator.push(&chunk(&rows, 1, 0, true)).unwrap().unwrap();
+        assert_series_eq(
+            Some(&series),
+            &vec![vec![Some(expected)]],
+            &output_metric(function, 0),
+        );
+    }
+}