Clean up PreserveLoggingContext (#7877)

This had some dead code and some just plain wrong docstrings.

changelog.d/7877.misc

@@ -0,0 +1 @@
+Clean up `PreserveLoggingContext`.

synapse/logging/context.py

@@ -566,36 +566,33 @@ class LoggingContextFilter(logging.Filter):
         return True
 
 
-class PreserveLoggingContext(object):
-    """Captures the current logging context and restores it when the scope is
-    exited. Used to restore the context after a function using
-    @defer.inlineCallbacks is resumed by a callback from the reactor."""
+class PreserveLoggingContext:
+    """Context manager which replaces the logging context
 
-    __slots__ = ["current_context", "new_context", "has_parent"]
+     The previous logging context is restored on exit."""
+
+    __slots__ = ["_old_context", "_new_context"]
 
     def __init__(
         self, new_context: LoggingContextOrSentinel = SENTINEL_CONTEXT
     ) -> None:
-        self.new_context = new_context
+        self._new_context = new_context
 
     def __enter__(self) -> None:
-        """Captures the current logging context"""
-        self.current_context = set_current_context(self.new_context)
-
-        if self.current_context:
-            self.has_parent = self.current_context.previous_context is not None
+        self._old_context = set_current_context(self._new_context)
 
     def __exit__(self, type, value, traceback) -> None:
-        """Restores the current logging context"""
-        context = set_current_context(self.current_context)
+        context = set_current_context(self._old_context)
 
-        if context != self.new_context:
+        if context != self._new_context:
             if not context:
-                logger.warning("Expected logging context %s was lost", self.new_context)
+                logger.warning(
+                    "Expected logging context %s was lost", self._new_context
+                )
             else:
                 logger.warning(
                     "Expected logging context %s but found %s",
-                    self.new_context,
+                    self._new_context,
                     context,
                 )