completionItem snippets conflict with signatureHelp

When implementing a java language server, it would be nice to be able to activate signatureHelp immediately, without a trigger character. Suppose for example the user has typed:

someValue.someMeth

We would like to autocomplete someValue.someMethod() and place the users cursor between parentheses. This is possible using snippets:

CompletionItem { label: someMethod, insertText: someMethod($0), insertTextFormat: 2 }

The problem is that signatureHelp relies on the trigger character (, and it doesn't get activated when the ( is inserted as part of the snippet.

This problem does not come up in languages like Typescript, because the expression someValue.someMethod is well-defined as a reference to someMethod. But in Java, method references have a different syntax someValue::someMethod. So we know the user will always type (, but we don't autocomplete it.

There are two possible solutions to this, one at the VSCode level and one at the language-server-protocol level:

1. VSCode solution: if the character before $0 in a snippet is a signatureHelp trigger character, activate signatureHelp.
2. LSP solution: add a property CompletionItem { ... activateSignatureHelp: boolean } that activates signature-help immediately following completion.

Start with the CompletionItem and signatureHelp behavior described in the issue, then compare the proposed VSCode-level and LSP-level approaches. Done means an agreed protocol or client behavior activates signatureHelp after a snippet inserts a trigger character, including the Java example.