Add support for multi-target exporter scraping

[SuperQ]

We should deprecate supporting multiple targets in the exporter with the multi-target exporter pattern.

This follows Prometheus best practices better.

[tpoindessous]

Hi !

You can see another example here : https://github.com/oliver006/redis_exporter

thanks !

[SuperQ]

@tpoindessous Care to make an implementation? The other PR author seems to have vanished.

[zerthimon]

Also interested in this feature. Currently I have to run separate prometheus-postgres-exporter process for every master and replicas.

[zerthimon]

While at it ) it would be nice if this could be configured in a prometheus job, to work with discovery, etc. This is implemented in redis-exporter:

- job_name: redis_exporter
   static_configs:
     - targets:
       - redis://redis1:6379
       - redis://redis2:6379
       - redis://redis-etc:6379
   metrics_path: /scrape
   relabel_configs:
     - source_labels: [__address__]
       target_label: __param_target
     - source_labels: [__param_target]
       target_label: instance
     - target_label: __address__
       replacement: prometheus-redis-exporter:9121

The redis-exporter is then answers on URIs like this http://prometheus-redis-exporter:9121/scrape?target=redis://redis1:6379