Monitor bluestore fragmentation score

[okozachenko1203]

problem

We faced osd up and down flipping issue last weekend. It was because of excessive fragment. As BlueStore operates, the free space on the underlying storage will become fragmented. This is normal and unavoidable, but excessive fragmentation causes slowdown.

solution

Expose fragment score as prometheus metric and configure alert rules for now.

ref: https://docs.ceph.com/en/latest/rados/operations/health-checks/?highlight=backfillfull%20ratio#bluestore-fragmentation

[ricolin]

(hints: run a simple tool that dumps all of that data, save to file, and use node_exporter feature to parse a file)

[ricolin]

so ceph actually got no trigger for this https://tracker.ceph.com/issues/55474