This sounds more like a use case for enhanced visibility or SearchableAttributes. If you are running Temporal with ElasticSearch I would highly recommend to use SearchableAttributes which allows you to list, filter, count executions based on your custom search criteria. It is much more powerful than just prometheus metric because it lets you list exact workflow executions based on your custom search criteria. Please see this forum thread for examples.
Regarding metric I’m not sure we want to expose this extensibility for dynamic tags mainly due to restrictions imposed by various metric backends. I would recommend to emit your custom metric using the metricScope on workflow context for this use case.