Node drain metadata

[nickethier]

When a node is drained in Nomad there is no way to tell who triggered the drain or why. This becomes more of a need as tools like the Nomad Autoscaler become more widely used.

One possible solution is to allow an option field or fields to be submitted with the node drain api call that are then shown in the node events.

[schmichael]

While it's tempting to add a generic emit a node event hook, I think a purpose built field for this is ideal so the UX is familiar:

git commit -m "We all know how to write commit messages"

nomad node drain -self -m "So let's add them to drains"

Not saying -m is the right option name, but I think that UX might be useful especially as more drains are orchestrated by code instead of humans.

[github-actions]

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