route_to_stream function does not reflect in stream messages/sec display

When using the pipeline route_to_stream function, messages sent to a new stream are not reflected in the monitoring of that stream on the streams page. The messages are going through the stream to the proper index, just the message rate shows '0'.

## Expected Behavior
The messages/second counter should show the actual messages/second through the stream regardless of source or insert point.

## Current Behavior
It seems messages sent from another stream via pipeline rules (route_to_stream) do not appear as a metric.

## Steps to Reproduce (for bugs)

1. Create stream A, Stream B; set different default indexes for each stream.
2. Use pipeline rules to route_to_stream from A to B
3. Validate the messages/sec on streams shows properly on A, but not on B.
4. Validate messages are still being stored in index B.

## Context
```
when
to_string($message._path) == "conn"
then
route_to_stream(id: "5c9901bb3b3e1ebc7103346d", remove_from_default: true);
end
```

## Your Environment
My processing order is changed from default:
1 Message filter chain
2 Pipeline Processor

* Graylog Version: 3.0.1