Description:
If the Agent can't login to the mysqld it is supposed to monitor, it can't get the UUID of the mysqld and thus can't (currently) report any problems to the Service Manager. It can only log them in the Agent's local error log, so the problem is not very visible to the user. We should allow the Agent to report such problems (e.g. if the login credentials are not correct) so it's easier for customers to see, diagnose, and fix certain problems. See EM-2953/Bug #37829 for background information.
How to repeat:
Option 1
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1) Install an agent and give it the wrong login credentials for the mysqld it is supposed to monitor.
2) Start the agent.
3) Notice that nothing shows up in the Dashboard -- there's only a message in the Agent's error log indicating the problem.
Option 2
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1) Get everything working nicely, with an Agent showing up in a Dashboard.
2) Change the user or password in the Agent's etc/instances/mysql/agent-instance.ini file.
3) Restart the agent.
4) Notice that Agent still shows as "down" in the Dashboard -- there's only a message in the Agent's error log indicating the problem.