Properly quote string arguments when displaying functions

[ndenev]

The Display implementation for Function is not properly quoting string arguments.
For example, doing parse_expr() on this query:

label_replace(metric{label1="value1",label2="value2",label3=~"value3.*"}, "new_label", "$1", "label3", ".*(([0-9]+)).*")

results in this:

label_replace(metric{label1="value1",label2="value2",label3=~"value3.*"}, new_label, $1, label3, .*(([0-9]+)).*)

And with the unquoted arguments, especially the regex reference $1 this fails to parse.

With this change the result is this:

label_replace(metric{label1="value1",label2="value2",label3=~"value3.*"}, "new_label", "$1", "label3", ".*(([0-9]+)).*")