Dataplane: Audit Prometheus heatmap changes in frontend

### What happened? Changes in https://github.com/grafana/grafana/pull/62694, changed the definition of heatmap fields on the backend, but these changes haven't yet been reflected on the frontend. We already patched the [primary symptom](https://github.com/grafana/grafana/issues/71017), in which heatmap formatted queries were throwing an error when the "Value" name was not present on the field. We need to audit all of the usages of "Value" as a dataFrame field name, and add documentation and tests as necessary to catch any remaining issues before our users do. This is an epic that will contain links to each individual usage. Related: https://github.com/grafana/grafana/pull/65237