[otlp] UseOtlpExporter and AddOtlpExporter extensions don't play nice together

In 1.8.0 we introduced the UseOtlpExporter extension which will turn on OtlpExporter for all signals. If UseOtlpExporter is called more than once, or AddOtlpExporter is called in addition, exceptions will be thrown:

• NotSupportedException: Multiple calls to UseOtlpExporter on the same IServiceCollection are not supported.

  https://github.com/open-telemetry/opentelemetry-dotnet/blob/57c442cc7b2fe93c81ca877de28f5204fdb11ebb/src/OpenTelemetry.Exporter.OpenTelemetryProtocol/Builder/OpenTelemetryBuilderServiceProviderExtensions.cs#L15

• NotSupportedException: Signal-specific AddOtlpExporter methods and the cross-cutting UseOtlpExporter method being invoked on the same IServiceCollection is not supported.

  https://github.com/open-telemetry/opentelemetry-dotnet/blob/57c442cc7b2fe93c81ca877de28f5204fdb11ebb/src/OpenTelemetry.Exporter.OpenTelemetryProtocol/Builder/OpenTelemetryBuilderServiceProviderExtensions.cs#L24

This was intentionally done but some users have run into issues:

• https://github.com/dotnet/aspire/pull/3697#issuecomment-2058028632

Opening this issue to track the demand for changing the behavior.