Only set log level for the root logger

Before this change we called `setLevel` on each logger with level set to
INFO. This meant that it was not possible to change log level for all
child modules by doing something like
`logging.getLogger('trino').setLevel(logging.DEBUG)` because the child
loggers had explicit levels set already. It instead required us to
change log levels for each module (`trino.client`, `trino.dbapi`,
`trino.auth` etc.) separately.

After this change only the root logger `trino` has a default level set.
Other child loggers inherit from it. So now the default log level for
all modules can be changed by doing
`logging.getLogger('trino').setLevel(logging.DEBUG)` for example.

trino/logging.py

@@ -11,12 +11,20 @@
 # limitations under the License.
 
 import logging
+from typing import Optional
 
 LEVEL = logging.INFO
 
 
 # TODO: provide interface to use ``logging.dictConfig``
-def get_logger(name: str, log_level: int = LEVEL) -> logging.Logger:
+def get_logger(name: str, log_level: Optional[int]) -> logging.Logger:
     logger = logging.getLogger(name)
-    logger.setLevel(log_level)
+    # We must not call setLevel by default except on the root logger otherwise
+    # we cannot change log levels for all modules by changing level of the root
+    # logger
+    if log_level is not None:
+        logger.setLevel(log_level)
     return logger
+
+# set default log level to LEVEL
+trino_root_logger = get_logger('trino', LEVEL)