Make `LLMEval` descriptors plottable from Tests

[elenasamuylova]

Issue description

• The TextLength() descriptor plot on the Descriptors tab does not populate when the TestShareOfOutRangeValues Test is logged to a Project. (The plot is visible but empty).
• Custom LLMEval() descriptor plot does not appear on the Descriptors tab when the TestCategoryCount or TestCategoryShare Test is logged to a Project. (The plot is not visible on the Descriptors tab + not plottable manually since no Metric is logged).

Steps to reproduce

• Run this notebook https://github.com/evidentlyai/community-examples/blob/main/tutorials/Regression_testing_with_debugging.ipynb
• In Evidently Cloud, add a new Tab - Descriptors

Expected behavior

• We can plot the distribution of the custom LLMEval() descriptor when running the Tests on category count/share. For Evidently Cloud, it is included automatically on the Descriptors tab.
• When running the test on the share of out-of-range values, we can see TextLength() plot on the Descriptors tab.