Support completer queries from SearchBackend

[kanatohodets]

This allows the SearchBackend to return query completions for virtual queries: e.g. virt.v1..servers-stat returns ["virt.v1.*.servers-status:"].

Why test for running a completion query using 'HasSuffix(query, "*")' rather than 'format == "completer"? The reason is that graphite-web translates format=completer from the javascript into format=json and a standard 'find' request, and it is this standard 'find' request which reaches carbonzipper.

[kanatohodets]

Some context for the comment about format changing:

From graphite-web, handling the AJAX request from the frontend for autocomplete:

  if format == 'completer':
    query = query.replace('..', '*.')
    if not query.endswith('*'):
      query += '*'

    if automatic_variants:
      query_parts = query.split('.')
      for i,part in enumerate(query_parts):
        if ',' in part and '{' not in part:
          query_parts[i] = '{%s}' % part
      query = '.'.join(query_parts)

  try:
    matches = list( STORE.find(query, fromTime, untilTime, local=local_only) )
  except:
    log.exception()
raise

The STORE.find at the end here will do a normal Find request to zipper, with the normal json or pickle format.

As a result, zipper never sees any 'completer' requests as far as I can tell. This is why we need to guess a bit about whether a query is a completer or not by looking for the *. However, I think this is a basically reasonable thing given how virtual metrics work -- normal globs aren't supported by carbonsearch, so the only concern is about colliding with a tag which legitimately has a glob at the end.