Switch task scheduler from raw logcontext manipulation (set_current_context) to utils (PreserveLoggingContext) (#18990)

Prefer the utils over raw logcontext manipulation.

Spawning from adding some logcontext debug logs in
#18966 and since we're not
logging at the `set_current_context(...)` level (see reasoning there),
this removes some usage of `set_current_context(...)`.

Author: MadLittleMods

changelog.d/18990.misc

@@ -0,0 +1 @@
+Switch task scheduler from raw logcontext manipulation to using the dedicated logcontext utils.

synapse/util/task_scheduler.py

@@ -27,8 +27,8 @@
 from synapse.logging.context import (
     ContextResourceUsage,
     LoggingContext,
+    PreserveLoggingContext,
     nested_logging_context,
-    set_current_context,
 )
 from synapse.metrics import SERVER_NAME_LABEL, LaterGauge
 from synapse.metrics.background_process_metrics import (
@@ -422,14 +422,11 @@ def _occasional_report(
             """
 
             current_time = self._clock.time()
-            calling_context = set_current_context(task_log_context)
-            try:
+            with PreserveLoggingContext(task_log_context):
                 usage = task_log_context.get_resource_usage()
                 TaskScheduler._log_task_usage(
                     "continuing", task, usage, current_time - start_time
                 )
-            finally:
-                set_current_context(calling_context)
 
         async def wrapper() -> None:
             with nested_logging_context(task.id) as log_context: