fs: Add metrics prometheus exporter

[metaspace]

Set environment variable ZENFS_EXPORT_PROMETHEUS=y when building to enable prometheus export of metrics. Exporter will listen on 127.0.0.1:8080.

Requires prometheus-cpp-pull == 1.1.0.

[metaspace]

LGTM. Can we update README (or add a new readme) with brief instructions to use this ?

@aravind-wdc Added section to README.md, please review :)

[yhr]

merge cmake changes first