Add metric ontology

[zhanyuanucb]

This PR fixes #566

Description of changes

Added forte/ontology_specs/metric.json

Possible influences of this PR.

Due to the ndarray attribute in NdMetric, the metric ontology spec isn't ready to be auto-generated by the current ontology generator yet.

Test Conducted

Unit tests are to be added

[hunterhector]

Could you do the following to make sure the PR is up to standard?

1. Change the PR name to the content instead of a number
2. Make the branch up to date to the branch
3. Make sure the CI passes.

[codecov]

Codecov Report

Merging #568 (0407b7a) into master (edd0f49) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #568   +/-   ##
=======================================
  Coverage   80.07%   80.07%           
=======================================
  Files         234      234           
  Lines       16538    16538           
=======================================
  Hits        13243    13243           
  Misses       3295     3295           

Impacted Files	Coverage Δ
forte/data/ontology/core.py	84.06% <ø> (ø)

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[hunterhector]

We normally also add the generated class to Forte repo so users don't have to re-generate these.