Get all problems (errors, warnings, etc.) using VSCode extensions API

I am creating a VSCode extension and I need a way to access all of the current errors, warnings, etc in the Problems pane but I am not even sure if the API provides access to this. I see that I can create Diagnostics and it appears that I can get those diagnostics with the DiagnosticCollection but I don't see where I can get a list of all of the errors and such. Does anyone have experience with this?

[sghung]

I am also a VSCode extension builder who wants to write automated tests for verifying a language server from the client side. Since I'm unable to programatically access the DiagnosticCollection, it is quite difficult to automate. Having additional API to access these problems would be very helpful.

[jrieken]

fyi @dbaeumer

[lggomez]

+1 to this

I'm writing an extension that performs actions based on json content and this would be very useful to make preemptive validations based on existing diagnostics