Allow pipe input

[fxnn]

Currently, sensu-prometheus-collector only accepts Prometheus text via an URL. However, in certain situations it is helpful to be able to pre-process the raw Prometheus data (e.g. via grep).

Therefore, for the flexibility of the tool, it would be great to be able to just pipe-in the Prometheus text.

(In my situation, an errorneous tool outputs a duplicate HELP line, which the underlying parser seems to reject. Would be easy to filter that via grep.)