Hello Mustafa Ispir
You can write a stored procedure which will perform the operations in a sql table if you want to append the data, I guess you are overwriting the file every time as the log file name is the same
In case of activity run duration, for instance for Copy activity you can pass the following to the sproc: @activity('COPY_ACTIVITY_NAME').output.copyDuration
This will pass the right duration to the sproc and it will perform append to the log table, you can use that log table in SQL to monitor the ETL logs, if you compulsorily need a file then you can write a custom code which will perform the append in the file (this will be kind of a transaction insert logic)