monitor: introduce Context.with_origin()

The existing JSONSeqMonitor was saving/restoring the "origin"
when generating a new log-entry. This allows logging from
different origins (e.g. "org.osbuild.main") in a kind of
"out-of-band" fashion.

But this save/restore feels slightly inelegant because
JSONSeqMonitor feels like the wrong layer to deal with this.
This is why a new `with_origin()` helper is introduced that
will either reuse the existing context or create a new one
with the requested origin.

osbuild/monitor.py

@@ -9,6 +9,7 @@ are called on the monitor object at certain events. Consult the
 """
 
 import abc
+import copy
 import datetime
 import hashlib
 import json
@@ -52,6 +53,18 @@ class Context:
         if name != "_id":
             super().__setattr__("_id", None)
 
+    def with_origin(self, origin: Optional[str]) -> "Context":
+        """
+        Return a Context with the given origin but otherwise identical.
+
+        Note that if the origin is empty or same it will return self.
+        """
+        if origin is None or origin == self._origin:
+            return self
+        ctx = copy.copy(self)
+        ctx.set_origin(origin)
+        return ctx
+
     @property
     def origin(self):
         return self._origin
@@ -322,13 +335,8 @@ class JSONSeqMonitor(BaseMonitor):
         self._context.set_stage(module)
 
     def log(self, message, origin: Optional[str] = None):
-        oo = self._context.origin
-        if origin is not None:
-            self._context.set_origin(origin)
-        entry = log_entry(message=message, context=self._context, progress=self._progress)
+        entry = log_entry(message, self._context.with_origin(origin), self._progress)
         self._jsonseq(entry)
-        # restore old origin
-        self._context.set_origin(oo)
 
     def _jsonseq(self, entry):
         # follow rfc7464 (application/json-seq)