Add README

README.md

+# CVMFS prometheus exporter
+
+The exporter will automatically discover the mounted cvmfs repositories using the 'cvmfs_config status' command.
+For all mounted repos the following stats will be published:
+* cvmfs_total_cache_size
+* cvmfs_cached_bytes
+* cvmfs_pinned_bytes
+* cvmfs_cache_hit_rate
+* cvmfs_pinned_bytes
+* cvmfs_expires
+* cvmfs_inode_max
+* cvmfs_maxfd
+* cvmfs_ncleanup24
+* cvmfs_nclg
+* cvmfs_ndiropen
+* cvmfs_ndownload
+* cvmfs_nioerr
+* cvmfs_nopen
+* cvmfs_pid
+* cvmfs_revision
+* cvmfs_rx
+* cvmfs_speed
+* cvmfs_timeout
+* cvmfs_timeout_direct
+* cvmfs_uptime
+* cvmfs_usedfd
+
+The description of each attribute stat can be found here:
+https://cvmfs.readthedocs.io/en/stable/cpt-details.html#getxattr
+
+All stats are reported (labeled) per cvmfs repository, for example:
+
+     HELP cvmfs_speed Shows the average download
+     TYPE cvmfs_speed
+     cvmfs_speed{cvmfs_repository="repo1.cern.ch"} 97.0
+     cvmfs_speed{cvmfs_repository="repo2.cern.ch"} 13.0
+
+## Configuring the exporter
+
+ - port: The port where the stats are published can be passed directly to the script as --port (-p) <value> or by setting the $PORT env variable (defaults to 8080).
+
+ - sampling interval: The sampling interval can be passed directly to the exporter as --sampling_interval (-s) <value_in_seconds> or by setting the $SAMPLING_INTERVAL env variable (defaults to 1).
+
+ - log level: The verbosity of the exporter can be passed to the script as --log_level (-l) <value> (critical, error, warning, info, debug) or by setting the $LOG_LEVEL env variable (defaults to info).
+
+## Grafana dashboard
+
+The json definition of an example grafana dashboard is included in this repo.