stdlib: add structlog.stdlib.ProcessorFormatter(use_get_message=True)

Fixes #520

CHANGELOG.md

@@ -34,6 +34,9 @@ You can find our backwards-compatibility policy [here](https://github.com/hynek/
 - `structlog.dev.RichTracebackFormatter` that allows to configure the traceback formatting.
   [#542](https://github.com/hynek/structlog/issues/542)
 
+- stdlib: `ProcessorFormatter` can now be told to now render the log record message using `getMessage` and just `str(record.msg)` instead.
+  [#550](https://github.com/hynek/structlog/issues/550)
+
 
 ### Fixed
 

src/structlog/stdlib.py

@@ -951,6 +951,10 @@ class ProcessorFormatter(logging.Formatter):
             This parameter exists for historic reasons. Please use *processors*
             instead.
 
+        use_get_message:
+            If True, use ``record.getMessage`` to get a fully rendered log
+            message, otherwise use ``str(record.msg)``. (default: True)
+
     Raises:
 
         TypeError: If both or neither *processor* and *processors* are passed.
@@ -963,6 +967,7 @@ class ProcessorFormatter(logging.Formatter):
     .. deprecated:: 21.3.0
        *processor* (singular) in favor of *processors* (plural). Removal is not
        planned.
+    .. versionadded:: 23.2.0 *use_get_message*
     """
 
     def __init__(
@@ -974,6 +979,7 @@ def __init__(
         keep_stack_info: bool = False,
         logger: logging.Logger | None = None,
         pass_foreign_args: bool = False,
+        use_get_message: bool = True,
         *args: Any,
         **kwargs: Any,
     ) -> None:
@@ -998,6 +1004,7 @@ def __init__(
         self.keep_stack_info = keep_stack_info
         self.logger = logger
         self.pass_foreign_args = pass_foreign_args
+        self.use_get_message = use_get_message
 
     def format(self, record: logging.LogRecord) -> str:
         """
@@ -1029,7 +1036,9 @@ def format(self, record: logging.LogRecord) -> str:
             logger = self.logger
             meth_name = record.levelname.lower()
             ed = {
-                "event": record.getMessage(),
+                "event": record.getMessage()
+                if self.use_get_message
+                else str(record.msg),
                 "_record": record,
                 "_from_structlog": False,
             }