Hydra

Related to bug 15466.

This merge request add kavlan & kwollect description for hydra.

assigned to @pijacquo

added 8 commits

• c609a09c...86026138 - 5 commits from branch master
• 920aa359 - [lyon][hydra] Put Hydra in kavlan
• c0277593 - [lyon][hydra] Describe Prometheus metrics
• a13005bf - [lyon][hydra] describe BMC metrics

Compare with previous version

merged

mentioned in commit 85e89dc1