Expose per org/instanceId active_series count

[woodsaj]

The total number of active_series on a MT instance is available, but for multi-tenant deployments there is no way to get the number of active_series per tenant (internally represented by OrgId)

It looks like this can be simply added by changing AggMetrics.Metrics from ```

map[schema.MKey]*AggMetric

to

map[uint32]map[schema.MKey]*AggMetric

All calls to AggMetrics methods already pass a schema.MKey which has an Org field that returns the OrgId

This change to AggMetrics will then allow us to keep perOrg MetricsActive counts.