Bump System.Diagnostics.DiagnosticSource from 8.0.1 to 9.0.0 in /docs/core/diagnostics/snippets/Metrics in the dotnet group

Bump System.Diagnostics.DiagnosticSource from 8.0.1 to 9.0.0 in /docs/core/diagnostics/snippets/Metrics in the dotnet group #2478