This allows the following expression syntaxes for selecting timeseries:
foo (already valid before)
foo{} (already valid before)
{job="prometheus"} (new, select all timeseries for job "prometheus")
Omitting both the metric name *and* any label matchers ("" or "{}") will
still yield a syntax error.
To get all timeseries, you could do:
{__name__=~".*"}
or, without relying on knowledge about __metric__:
{job=~".*"}
Change-Id: Ifee000b9ac0184ef6ced18411069c7f2699a2dda
0e48c18bbf