Support a metric being a string value as well as a numeric value

[epugh]

In working with @frejonb on using the RAGElo project (https://github.com/zetaalphavector/RAGElo) we found that sometimes a the query and overall score level we want a string value, not a number.

For example, at the per query level, we wanted to know which agent was the winner, A or B. Yes, we could have modeled that as a 0 or a 1, but that is awkward.

We also discovered that the assumption that at the top level we would just roll up the average of all query level metrics was flawed. You can't average a string, so in our POC we just took the first value as the value to show at the Experiment level.

This PR doesn't touch on the idea that we need to store Experiment level metrics at the Experiment level instead of just averaging all the query level data.

Description

If a value is a String, treat it as String.

Issues Resolved

[List any issues this PR will resolve]

Check List

• [ x] New functionality includes testing.
  • [ x] All tests pass, including unit test, integration test
• New functionality has been documented.
• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.