enhancement(codecs): Add enhanced metric tag option to json and text encoders (#15716)

* The `vrl-lib` dep is no longer optional for `vector-core`

* Move `MetricTagValues` to the codecs lib

* enhancement(codecs): Add enhanced metric tag option to text encoder

* Refactor tag reduction to single values and add proptest

* enhancement(codecs): Add enhanced metric tag option to json encoder

* Skip serializing the new `metric_tag_values` if default