"area" diagnostics is missing cell_methods in fv_diagnostics.F90 causing postprocessing issues

[nikizadehgfdl]

Describe the bug
The "area" diagnostics on line 417 of fv_diagnostics.F90 is missing the cell_methods call below:

if (id_area > 0)  id_dx = register_static_field( trim(field), 'dx', (/id_xt,id_y/),  call diag_field_add_attribute (id_area, 'cell_methods', 'area: sum')

The call was there but was removed (probably by mistake) in commit b223632.

This is causing issues in postprocessing (frepp) tools at gfdl.

To Reproduce
The issues are reproduce only through frepp tools.
User reports bad results for "area" when regridded from CS to latlon (e.g., ESM4_amip_D1_am4p2_proto7b (dora #1669).)

Expected behavior
static "area" diagnostics should have cell_methods attribute.

[laurenchilutti]

This fix has been merged in.