Sample oslo.metrics codes

This patch is a sample oslo.metrics code to gather oslo.messaging's
metrics and export the metrics to prometheus. Some parts of this code
doesn't follow OpenStack standards style, and this lacks test codes.
Please use this patch as PoC for oslo.metrics.

Co-Authored-By: Yuki Nishiwaki <yuki.nishiwaki@linecorp.com>
Change-Id: I9434d11466e7626fdbebd1340a8bb3d664518bd1