Adding logging statements at start and end of all stored procedures
I would advise against programmatically modifying all the stored procedures in a database like that.  Trying to ensure no edge-case causes a modification to be made erroneously will prove quite difficult. 

Instead of modifying all the stored procedures, why not use Extended Events to capture the information you're interested in.  This will


Not impact performance nearly as much
Allow you to enable/disable monitoring as required, without modifying any stored procedures.
Not affect execution plans for the stored procedures.


SQLTips has a good example of several ways to capture stored procedure execution history without modifying the stored procedures.