Documentation for streetside imagery can be found here: https://zcusa.951200.xyz/en-us/bingmaps/articles/getting-streetside-tiles-from-imagery-metadata
As for the label layer, if it is not documented as an imagerySet
here you should not be accessing it. There is no documentation as it's not a supported layer to access.
That said, if you have the option, I highly recommend using Azure Maps instead. It's the same data, much easier to license, has increased security options, and has additional tile layers such as a label only layer, weather, and traffic layers. There is even a library to make it easier to integrate with OpenLayers: https://github.com/Azure-Samples/azure-maps-openlayers