Fix run_as_background_process not following Synapse log context rules

Resulting in bad logs being seen:

```
PreserveLoggingContext: Expected logging context sentinel but found main
```

```
LoggingContext: Expected logging context main was lost
```

Author: MadLittleMods

synapse/metrics/background_process_metrics.py

@@ -47,7 +47,7 @@
 from synapse.logging.context import (
     ContextResourceUsage,
     LoggingContext,
-    PreserveLoggingContext,
+    make_deferred_yieldable,
 )
 from synapse.logging.opentracing import SynapseTags, start_active_span
 from synapse.metrics import SERVER_NAME_LABEL
@@ -223,10 +223,9 @@ def run_as_background_process(
     This should be used to wrap processes which are fired off to run in the
     background, instead of being associated with a particular request.
 
-    It returns a Deferred which completes when the function completes, but it doesn't
-    follow the synapse logcontext rules, which makes it appropriate for passing to
-    clock.looping_call and friends (or for firing-and-forgetting in the middle of a
-    normal synapse async function).
+    It returns a Deferred which completes when the function completes, which makes it
+    appropriate for passing to clock.looping_call and friends (or for
+    firing-and-forgetting in the middle of a normal synapse async function).
 
     Args:
         desc: a description for this background process type
@@ -241,8 +240,6 @@ def run_as_background_process(
 
     Returns:
         Deferred which returns the result of func, or `None` if func raises.
-        Note that the returned Deferred does not follow the synapse logcontext
-        rules.
     """
 
     async def run() -> Optional[R]:
@@ -280,10 +277,9 @@ async def run() -> Optional[R]:
                     name=desc, **{SERVER_NAME_LABEL: server_name}
                 ).dec()
 
-    with PreserveLoggingContext():
         # Note that we return a Deferred here so that it can be used in a
         # looping_call and other places that expect a Deferred.
-        return defer.ensureDeferred(run())
+        return make_deferred_yieldable(defer.ensureDeferred(run()))
 
 
 P = ParamSpec("P")