Sqlserver receiver should allow default instances on Windows

[MikkelPorse]

Component(s)

receiver/sqlserver

Is your feature request related to a problem? Please describe.

If you run a default installation of SQL Server (ie. no named instance) - you're limited to the non-performance counter version of the receiver

Describe the solution you'd like

I'd like to be able to specify only computer_name and not instance_name and have the receiver get performance counters from Windows

Describe alternatives you've considered

I could just use the performance counter receiver, but I'd need to specify which by hand - a ready-built sqlserver receiver is more handy :)

Additional context

related to this PR #31915

[github-actions]

Pinging code owners:

• receiver/sqlserver: @djaglowski @StefanKurek

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