[Microsoft SQLServer] Add Multi-host Support

[muthu-mps]

Summary

• The MSSQL server can have multiple instance configured on the same machine which is accessed using the named_instance connection string. We can even launch multiple hosts separately.
• The beats modules mssql metricbeat provides option to configure multiple hosts and fetch metrics from different hosts. The same behaviour is verified with the Generic sql module with which we are able to configure multiple hosts and fetch metrics.
• When it comes to the agent based integrations we have to add multiple integrations to fetch metrics from different hosts. Either it is hosted as a named_instance or as a separate host.
• The integrations configuration should be tweaked to support metrics collection from multiple hostsusing a single integration instead of adding multiple integrations.

[nerophon]

Specific example:

We have around 5000 MSSQL servers, each with multiple SQL instances running on them. We can't call out each instance by name, we need a method of using a wildcard or some sort of discovery that picks up the instances on the SQL server In one example we use perfmon counters. They are prefixed with the SQL instance name, and in this case there are 2 instance names across the various counters. I need performance metrics from both instances on this particular host. In this case, we aren't even using the MSSQL integration, rather it's the `Windows` integration using the performance counters metrics and targeting SQL specific counters.

Please let me know if you require further explanation of this example use-case.