feat: memtable stats (#1591)

* feat: memtable stats

* chore: add tests for timestamp subtraction

* feat: add `Value:as_timestamp` method

src/common/time/src/timestamp.rs

@@ -48,7 +48,7 @@ impl Timestamp {
     /// The result time unit remains unchanged even if `duration` has a different unit with `self`.
     /// For example, a timestamp with value 1 and time unit second, subtracted by 1 millisecond
     /// and the result is still 1 second.
-    pub fn sub(&self, duration: Duration) -> error::Result<Self> {
+    pub fn sub_duration(&self, duration: Duration) -> error::Result<Self> {
         let duration: i64 = match self.unit {
             TimeUnit::Second => {
                 i64::try_from(duration.as_secs()).context(TimestampOverflowSnafu)?
@@ -79,6 +79,13 @@ impl Timestamp {
         })
     }
 
+    /// Subtracts current timestamp with another timestamp, yielding a duration.
+    pub fn sub(&self, rhs: Self) -> Option<chrono::Duration> {
+        let lhs = self.to_chrono_datetime()?;
+        let rhs = rhs.to_chrono_datetime()?;
+        Some(lhs - rhs)
+    }
+
     pub fn new(value: i64, unit: TimeUnit) -> Self {
         Self { unit, value }
     }
@@ -863,19 +870,19 @@ mod tests {
     #[test]
     fn test_timestamp_sub() {
         let res = Timestamp::new(1, TimeUnit::Second)
-            .sub(Duration::from_secs(1))
+            .sub_duration(Duration::from_secs(1))
             .unwrap();
         assert_eq!(0, res.value);
         assert_eq!(TimeUnit::Second, res.unit);
 
         let res = Timestamp::new(0, TimeUnit::Second)
-            .sub(Duration::from_secs(1))
+            .sub_duration(Duration::from_secs(1))
             .unwrap();
         assert_eq!(-1, res.value);
         assert_eq!(TimeUnit::Second, res.unit);
 
         let res = Timestamp::new(1, TimeUnit::Second)
-            .sub(Duration::from_millis(1))
+            .sub_duration(Duration::from_millis(1))
             .unwrap();
         assert_eq!(1, res.value);
         assert_eq!(TimeUnit::Second, res.unit);
@@ -914,4 +921,17 @@ mod tests {
             Timestamp::new(1, TimeUnit::Second).to_local_string()
         );
     }
+
+    #[test]
+    fn test_subtract_timestamp() {
+        assert_eq!(
+            Some(chrono::Duration::milliseconds(42)),
+            Timestamp::new_millisecond(100).sub(Timestamp::new_millisecond(58))
+        );
+
+        assert_eq!(
+            Some(chrono::Duration::milliseconds(-42)),
+            Timestamp::new_millisecond(58).sub(Timestamp::new_millisecond(100))
+        );
+    }
 }