Consider using TryAddSingleton for services that users can override

[BrennanConroy]

For our public services like IUserIdProvider,IHubActivator, HubLifetimeManager, and IHubProtocolResolver we should be using TryAddSingleton so that if a user adds the service before calling AddSignalR we don't override their setting.

SignalR/src/Microsoft.AspNetCore.SignalR.Core/SignalRDependencyInjectionExtensions.cs

Lines 23 to 30 in 948ebf3

	services.AddSingleton(typeof(HubLifetimeManager<>), typeof(DefaultHubLifetimeManager<>));
	services.AddSingleton(typeof(IHubProtocolResolver), typeof(DefaultHubProtocolResolver));
	services.AddSingleton(typeof(IHubContext<>), typeof(HubContext<>));
	services.AddSingleton(typeof(IHubContext<,>), typeof(HubContext<,>));
	services.AddSingleton(typeof(HubConnectionHandler<>), typeof(HubConnectionHandler<>));
	services.AddSingleton(typeof(IUserIdProvider), typeof(DefaultUserIdProvider));
	services.AddSingleton(typeof(HubDispatcher<>), typeof(DefaultHubDispatcher<>));
	services.AddScoped(typeof(IHubActivator<>), typeof(DefaultHubActivator<>));

@davidfowl Did we have a reason for using AddSingleton or did we just forget?

Technically this would be a breaking change, but in a good, probably acceptable, way.

[davidfowl]

We forgot, seems like a fine thing to change in 2.2. I'm not sure we need to patch it but we can if there's evidence that people are seeing problems.

[mikaelm12]

Done: e41764c