Prometheus exporter should export names as labels

## What?

When customers or users are attempting to use the Prometheus endpoint, it's proving difficult to reference a stream or input in a way that's human readable. E.g., I get an ID, but have no way of referencing the name:

Ideally, those seeing to graph the metrics would have a way of readily identifying things like:
* Inputs
* Streams
* Nodes
* Rules
* Pipelines

So where I might have a stream ID exposed as a label, another label would be `stream_name`, `input_name`, `rule_name` etc. This would seem to fall in line with [Prometheus label name conventions](https://prometheus.io/docs/practices/naming/), as well as [Prometheus' data model](https://prometheus.io/docs/concepts/data_model/#metric-names-and-labels)

Implementing this pattern would go a long way to enabling customers and users to use the exported metrics in meaningful ways to observe and monitor their Graylog deployments.