fix(promql): preserve ordinary NaN samples (#8494)

* fix(promql): distinguish stale markers from NaN

Signed-off-by: discord9 <discord9@163.com>

* fix(promql): preserve ordinary NaN samples

Signed-off-by: discord9 <discord9@163.com>

---------

Signed-off-by: discord9 <discord9@163.com>

src/common/query/src/lib.rs

@@ -17,6 +17,7 @@ pub mod error;
 pub mod logical_plan;
 pub mod native_histogram;
 pub mod prelude;
+pub mod prometheus;
 pub mod request;
 pub mod stream;
 #[cfg(any(test, feature = "testing"))]

src/common/query/src/prometheus.rs

@@ -0,0 +1,37 @@
+// Copyright 2023 Greptime Team
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+/// Canonical Prometheus stale-marker NaN bit pattern.
+pub const PROMETHEUS_STALE_NAN_BITS: u64 = 0x7ff0_0000_0000_0002;
+
+/// Returns whether `value` is the canonical Prometheus stale-marker NaN.
+#[inline]
+pub fn is_prometheus_stale_nan(value: f64) -> bool {
+    value.to_bits() == PROMETHEUS_STALE_NAN_BITS
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn recognizes_only_the_canonical_stale_marker() {
+        assert!(is_prometheus_stale_nan(f64::from_bits(
+            0x7ff0_0000_0000_0002
+        )));
+        assert!(!is_prometheus_stale_nan(f64::from_bits(
+            0x7ff8_0000_0000_0000
+        )));
+    }
+}