Fix comments and exception message for metrics_to_scalars

[edward-io]

What does this PR do?

Fix comments and exception string for metrics_to_scalars, which can return non-float values, such as int or complex, depending on the dtype of the tensor.

Does your PR introduce any breaking changes? If yes, please list them.

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[codecov]

Codecov Report

Merging #8782 (b91993b) into master (e541803) will decrease coverage by 4%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #8782    +/-   ##
=======================================
- Coverage      93%     89%    -4%     
=======================================
  Files         169     169            
  Lines       14070   14070            
=======================================
- Hits        13040   12462   -578     
- Misses       1030    1608   +578