Add ability to send metrics for etcd-diego

- `etcd-diego` is the new name for the `etcd_metrics_server` used by diego.
- Refactored to remove smelly parse-time construction of handler mapping hash. Move to a saner way.
- Eliminated the global hash but left the instance map. At some stage we should look to remove this one as well, perhaps moving it to an instance of the Handler.
- Some initialization ordering changes so the classes are visible to the handler mappings builder code.

Note this is in Diego backlog - https://www.pivotaltracker.com/projects/1003146

[#69462832]

Signed-off-by: Chris Adams <cadams@pivotallabs.com>