feat: use already calculated exc_text

The logging module sets the traceback string to `record.exc_text` and
doesn't caluclate it again.
This allows also to log exceptions, when we are not in the stack-context
anymore and have no `exc_info` tuple.
For example, in some cases, only the textual representation of a
traceback is stored because the traceback object would be a memory leak
and prevent garbage collection.

Use case is therefor:
`formatter.format(record)` where `exc_text` was already created by
another earlier formatter or filter.

Author: spaceone

src/logfmter/formatter.py

@@ -106,10 +106,7 @@ def format_exc_info(cls, exc_info: ExcInfo) -> str:
             except Exception:
                 logging.exception()
         """
-        # Tracebacks have a single trailing newline that we don't need.
-        value = "".join(traceback.format_exception(*exc_info)).rstrip("\n")
-
-        return cls.format_string(value)
+        return "".join(traceback.format_exception(*exc_info))
 
     @classmethod
     def format_params(cls, params: dict) -> str:
@@ -242,10 +239,15 @@ def format(self, record: logging.LogRecord) -> str:
         if formatted_params:
             tokens.append(formatted_params)
 
-        if record.exc_info:
+        if record.exc_info and not record.exc_text:
             # Cast exc_info to its not null variant to make mypy happy.
             exc_info = cast(ExcInfo, record.exc_info)
-            tokens.append(f"exc_info={self.format_exc_info(exc_info)}")
+            record.exc_text = self.format_exc_info(exc_info)
+
+        if record.exc_text:
+            # Tracebacks have a single trailing newline that we don't need.
+            exc_text = record.exc_text.rstrip("\n")
+            tokens.append(f"exc_info={self.format_string(exc_text)}")
 
         if record.stack_info:
             stack_info = self.formatStack(record.stack_info).rstrip("\n")