Add SystemClock (#8239)

This PR adds a `SystemClock` trait for abstracting away the system
clock.

This allows us to swap out the real system clock with a
`FakeSystemClock` in the tests, thus allowing the fake passage of time.

We're using this in `Telemetry` to better mock the clock for testing
purposes.

Release Notes:

- N/A

crates/clock/src/system_clock.rs

@@ -0,0 +1,59 @@
+use chrono::{DateTime, Utc};
+
+pub trait SystemClock: Send + Sync {
+    /// Returns the current date and time in UTC.
+    fn utc_now(&self) -> DateTime<Utc>;
+}
+
+pub struct RealSystemClock;
+
+impl SystemClock for RealSystemClock {
+    fn utc_now(&self) -> DateTime<Utc> {
+        Utc::now()
+    }
+}
+
+#[cfg(any(test, feature = "test-support"))]
+pub struct FakeSystemClockState {
+    now: DateTime<Utc>,
+}
+
+#[cfg(any(test, feature = "test-support"))]
+pub struct FakeSystemClock {
+    // Use an unfair lock to ensure tests are deterministic.
+    state: parking_lot::Mutex<FakeSystemClockState>,
+}
+
+#[cfg(any(test, feature = "test-support"))]
+impl Default for FakeSystemClock {
+    fn default() -> Self {
+        Self::new(Utc::now())
+    }
+}
+
+#[cfg(any(test, feature = "test-support"))]
+impl FakeSystemClock {
+    pub fn new(now: DateTime<Utc>) -> Self {
+        let state = FakeSystemClockState { now };
+
+        Self {
+            state: parking_lot::Mutex::new(state),
+        }
+    }
+
+    pub fn set_now(&self, now: DateTime<Utc>) {
+        self.state.lock().now = now;
+    }
+
+    /// Advances the [`FakeSystemClock`] by the specified [`Duration`](chrono::Duration).
+    pub fn advance(&self, duration: chrono::Duration) {
+        self.state.lock().now += duration;
+    }
+}
+
+#[cfg(any(test, feature = "test-support"))]
+impl SystemClock for FakeSystemClock {
+    fn utc_now(&self) -> DateTime<Utc> {
+        self.state.lock().now
+    }
+}