Use SystemClock in EventCoalescer (#8317)

This PR updates the `EventCoalescer` to use the `SystemClock` trait to
abstract over the clock.

This allows us to test the advancement of time without relying on the
caller passing in the current time.

Release Notes:

- N/A

crates/client/src/telemetry.rs

@@ -96,7 +96,7 @@ impl Telemetry {
             log_file: None,
             is_staff: None,
             first_event_date_time: None,
-            event_coalescer: EventCoalescer::new(),
+            event_coalescer: EventCoalescer::new(clock.clone()),
             max_queue_size: MAX_QUEUE_LEN,
         }));