x/tools/gopls: add log statement to indicate gopls is serving

[stamblerre]

Running gopls on the command-line directly currently defaults to serve mode, which is unhelpful for users of the CLI and unintuitive. We should add a log message indicating that gopls is serving and how to get help text.

[gopherbot]

Change https://golang.org/cl/260008 mentions this issue: goLanguageServer: use gopls serveinstead of the defaultgopls``

[stamblerre]

@findleyr and @ianthehat have discussed that most language servers do default to serve, but we can improve the UX by adding a log statement. So I will repurpose this issue to add a log statement indicating that gopls is serving.

[mattn]

FYI, as far as that I have looked over 70 language servers in my project https://github.com/mattn/vim-lsp-settings, most language server does not provide serve command.