This plugin shows type annotations for functions in virtual text using built-in LSP client.
I use it with ocaml-lsp, but it will probably work with other LSP servers that support textDocument/codeLens request.
Install with plugin manager:
Plug 'jubnzv/virtual-types.nvim'
And add the following line in your LSP configuration:
lua require'nvim_lsp'.ocamllsp.setup{on_attach=require'virtualtypes'.on_attach}