Conditionally remove instrument caching in Metrics source-gen

Right now we have caching of instruments in Metrics source-gen:
https://github.com/dotnet/extensions/blob/7271d1923ef31670feefbcba1e55751dcc3e6bfa/src/Generators/Microsoft.Gen.Metrics/MetricFactoryEmitter.cs#L109

The original issue https://github.com/dotnet/runtime/issues/84713 was resolved in `System.Diagnostics.DiagnosticSource` of version 8, thus we can check its version during generation time and omit adding any caching logic. If the package version is older than 8, we will produce the same codebase as we have now.