merge meters and group menus

[huss]

Is your feature request related to a problem? Please describe.

The distinction of meters and groups is less clear to users and often an artifact of what meters collection.

Describe the solution you'd like

The basic idea is to fuse meters and groups menus per the merge meter and group menus Design Doc.

Describe alternatives you've considered

Leave UI as is with the issues discussed in the document.

Additional context

When people were asked to comment on this, only a few positive comments were made. Given this is a non-trivial UI change, doing it sooner rather than later is desirable.