No label suggestions were being returned for multi-line aggregation
contexts because the parsed selector string does not see the full
context before a `by` or `without` clause.

This solution stitches together all text nodes that comprise the query
editor to ensure the selector has sufficient context to generate
suggestions.

Also, an additional workaround has been included to ensure range vector
syntax does not disrupt label suggestions in aggregation contexts.

Related: #12890