Allow omitting the metric name in queries.

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