Is there a limit on the metrics monitored for all the application monitors like IIS and SQL?
Yes, for every application monitored, there is a limit on the number of metrics monitored within the application. The given limit is for a single monitor. 
Monitor  | Type  | Licensing Limit  | 
 | Basic | 100 sites, 100 applications, 100 application pools. For additional monitoring support, contact support@site24x7.com 
  | 
 | Advanced  | 200 databases  | 
 | Basic  | 100 containers, 100 images  | 
 | Advanced  | 100 host instances, 100 message box hosts, 100 message box general, 100 orchestrations, 100 send ports, 100 receive ports, 100 hosts, 100 receive locations  | 

You need to have a 
Windows/
Linux agent installed to monitor any of the above applications.
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