Support metric probes with no labels

[tommagowan]

The following probe, with no metricLabels field:

  - name: noLabels
    type: exists
    query: SELECT 1 FROM DUAL
    metricName: no_labels

Yields the following prometheus metric:
no_labels{...,null}

Prometheus does not parse this properly and reports a 'no token found' error. This causes all metrics to be dropped by Prometheus.