Provide a hook to call `OpenTelemetry.SuppressInstrumentationScope.Begin`

[nblumhardt]

Usage:

Log.Logger = new LoggerConfiguration()
    .WriteTo.OpenTelemetry(options =>
    {
        options.OnBeginSuppressInstrumentation =
            OpenTelemetry.SuppressInstrumentationScope.Begin;
        // ...

Fixes #41