Real-time gauge metrics would be useful

# Summary

It isn't possible to emit a real-time gauge metric for counting how many of a workflow are currently active. And probably other things than just workflow.duration which is the only possible one right now.

## Use Cases

If you'd like a prometheus metric per workflow category (ignoring how we define categories) you must place a dummy task at the start of a workflow run to emit that counter. It would be far saner to just emit it as a workflow level counter metric.

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**Message from the maintainers**:

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Start by tracing how the existing workflow.duration metric is emitted and how workflow-level metrics are defined. Clarify which real-time gauge metrics and workflow categories are required, then determine the tests needed to verify their values and lifecycle behavior.

Written by the indexing model from the issue text.