add log message to _prepare()

Author: dmig

asyncpg/connection.py

@@ -567,6 +567,8 @@ async def _prepare(
         use_cache: bool=False,
         record_class=None
     ):
+        if self._query_logger:
+            self._query_logger.debug('Preparing query: %s', query)
         self._check_open()
         stmt = await self._get_statement(
             query,