[chore] sqlserverreceiver/Add run as admin to docs (open-telemetry#27452

)

**Description:** 
Added a line explaining to run collector as admin, since not all perf
counters are collected unless you do.
(Or I'm missing something, which should be documented)

**Testing:** No unit tests, but observed this with a simple collector
install on windows.

**Documentation:** See above.

receiver/sqlserverreceiver/README.md

@@ -17,6 +17,8 @@
 The `sqlserver` receiver grabs metrics about a Microsoft SQL Server instance using the Windows Performance Counters.
 Because of this, it is a Windows only receiver.
 
+Make sure to run the collector as administrator in order to collect all performance counters for metrics. 
+
 ## Configuration
 
 The following settings are optional: