[retried_tests.ml]: Add README and refactor
Describe this metric in README.md
and do a slight refactoring.
The updated README is rendered here: https://gitlab.com/nomadic-labs/tezos-sw-metrics/-/tree/arvid@retried_tests-readme-refactor#retried_testsml-retried-flaky-tests
Edited by Arvid Jakobsson