[poll📊] UI preference on adding new value to Explore control

[junlincc]

Background

Some users found that having "+" at the top right corner in the control select field is unnecessary and somewhat confusing when only one selected option is allowed (e.g. charts allowing only a single metric). This is a new change implemented in Explore 1.0.

Some users have also struggled to add a second metric since the most prominent "+ Add metric" CTA would disappear when they added the first metric:

Proposed changes:

Instead of greyout the "+" on the top-right corner when metric limit is reached and hide the "+ Add metric/filter" CTA when first metric is added, we remove "+" at the top-right corner and but always keep "+ Add metric/filter" (the ghost button) when users can still add a new metric.

related #12631

Please join the poll, and let us know if you have even better solutions!