Some people use the Glass command line client to integrate with Emacs/Vim/VSCode. Internally we have an LSP server that queries Glass, but it's not open source and some work would be needed to extract it. The only non trivial thing it does is position mapping to account for local changes.
The integrations for code review and symbol search are both built for internal tools and not amenable to open sourcing.
FWIW I agree that the lack of open source integrations are the main barrier for external adoption