add option to disable metric suffixes

[zeitlinger]

Fixes #1555

[fstab]

I think we should not continue with this before we have consensus in the Prometheus team on whether the exposition format should be determined at deploy time, or at scrape time via the Accept header. I put this on the agenda for the Prometheus dev summit.

If we decide on a deploy time config for suffixes we should at least allow users to explicitly use a suffix if they want. If a user explicitly creates a counter named requests_total it should not be renamed to requests, independent of config options.

Also note that some metric types like Histograms and Summaries will still require suffixes in text format.