The Flow Datasource's only interface-based filter is withIfIndex(Integer ifIndex).

None of the other Helm datasources return, or can be filtered to return, an ifIndex.

Flow DS filters should be able to accept entities returned by other Helm DS queries as template variables.

Steps to Reproduce:

1. Create a new dashboard with template variables for location, exporter, interfaceResource, exporterNodeId, and interfaceIfIndex

   1. location is a query against Flow DS: locations()

   2. exporter is a nodeFilter query against Flow DS: nodeFilter(location = '$location' and nodeLabel = '<label of exporter node>'

   3. interfaceResource is a nodeResources query against Flow DS: nodeResources($exporter, name, interfaceSnmp)

   4. exporterNodeId is a postgresql query against the OpenNMS database for SELECT nodeid FROM node WHERE foreignsource = split_part('$exporter',':',1) AND foreignid = split_part('$exporter',':',2);

   5. interfaceIndex is a postgresql query against the OpenNMS database for SELECT iface.snmpifindex FROM snmpinterface AS iface WHERE iface.nodeid = $exporterNodeId AND iface.snmpifname = '<name of interface>';

2. Create a Time Series graph panel with a query against the flow datasource for applications includeOther() topN(0) withExporterNode($exporter) withGroupByInterval(1m) perSecond() toBits() withIfIndex($interfaceResource)

   1. Observe that this query fails, because the flow datasource cannot resolve the interface from the resource name, id, or label.

3. Change the argument of withIfIndex to $interfaceIfIndex

   1. Observe that the query works, because an integer ifIndex is supplied.