fix metrics

[Keksoj]

For now, when we query metrics… nothing appears. Also metrics enabling and disabling doesn't work.

Let's fix it.

• [x] fix "metrics enable" and disable and clear
• [x] fix "query metrics" to display something

This is done, but… it appears @Geal did not finish to implement all intended features on metrics. Left to do:

• [x] recreate a proper general output for all metrics (percentiles, nice tables)
• [ ] decomment and bring to life the commented code of dump_cluster_data()
• [ ] check if the cluster and backend ids provided in the command line actually exist, send an error if not.