metric-backend: Unify the query form layout across designers

The chained "then" steps rendered as table rows, forcing the
custom-query special agent form into a table layout whose labels sat
beside the inputs, while every other section in the graph designer
stacked each label above its field.

Render the then steps as a stack and move the custom-query form onto the
designer's shared layout, so both query forms present the same
vertically stacked layout.

Change-Id: Ibfcb06c22b111e0ad45da7d5eef6934f97863618