Empty `group_left`/`group_right` isn't working in rules

[beorn7]

I still have to understand why, but a group_left or group_right clause without any labels creates an evaluation error if in a rule:

WARN[0173] Error while evaluating rule "fake_rule{} = (chef_client_errors_count > 0) * ON(instance) GROUP_RIGHT() alerting_contact\n": parse error at char 59: unexpected ")" in grouping opts, expected identifier  source=manager.go:265

It appears the parser is adding an empty () at some point, which is then invalid syntax.

(In different news, group_right() should probably be legal and equivalent to group_right.)

@brian-brazil @fabxc

[brian-brazil]

This is another outcome of alert rules parsing the string version of the rules.

I plan on making an empty list of labels valid, but there's a new feature I want to go with it (see #1597).

[beorn7]

So we need to fix the stringification of a PromQL expression.

Work around for now is to use group_left(dummy)...

[lock]

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