Store: introduce metric for delayed storage maintenance target

This simple MR aims to add a new metrics that will reflect the value of the next storage maintenance to occur.

For now, the dashboard is not that useful as, by default, there is no maintenance delay. As soon as their will be, it will allow one to know if some hardware spikes are related to a maintenance.

We can imagine a better metric that reflects the blocks prior to the maintenance acting as a countdown, but I feel that this simple version is enough. Comments are welcome.

Manually testing the MR

Not much to check here. You can have a look to my custom dashboard that uses it http://platypus.testnets.tzalpha.net:3000/d/j2yqcTenk/tezos-full-dashboard-branch3a-master?orgId=1&from=now-5m&to=now . It has been added very recently so you need to wait a bit (next cycle) to see an actual level set.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR