Add support for the rdkafka 'stats_cb' config option in get_journal_client

this options allows to define a callback which will be called once every
'statistics.interval.ms' ms by rdkafka with a bunch of statistics (as a
json string).

See https://github.com/edenhill/librdkafka/blob/master/STATISTICS.md

This allows to define this callback as a string of the form:

  "path.to.module:function"

swh/journal/client.py

@@ -4,6 +4,7 @@
 # See top-level LICENSE file for more information
 
 from collections import defaultdict
+from importlib import import_module
 import logging
 import os
 from typing import Any, Callable, Dict, List, Optional, Set, Tuple, Union
@@ -34,6 +35,30 @@ def get_journal_client(cls: str, **kwargs: Any):
     Currently, only the "kafka" journal client is supported.
     """
     if cls == "kafka":
+        if "stats_cb" in kwargs:
+            stats_cb = kwargs["stats_cb"]
+            if isinstance(stats_cb, str):
+                try:
+                    module_path, func_name = stats_cb.split(":")
+                except ValueError:
+                    raise ValueError(
+                        "Invalid stats_cb configuration option: "
+                        "it should be a string like 'path.to.module:function'"
+                    )
+                try:
+                    module = import_module(module_path, package=__package__)
+                except ModuleNotFoundError:
+                    raise ValueError(
+                        "Invalid stats_cb configuration option: "
+                        f"module {module_path} not found"
+                    )
+                try:
+                    kwargs["stats_cb"] = getattr(module, func_name)
+                except AttributeError:
+                    raise ValueError(
+                        "Invalid stats_cb configuration option: "
+                        f"function {func_name} not found in module {module_path}"
+                    )
         return JournalClient(**kwargs)
     raise ValueError("Unknown journal client class `%s`" % cls)