Multi-Target Exporter Pattern

[neufeldtech]

See #34 for some additional context.

Consider adopting the multi-target exporter pattern for querying multiple Solidfire instances with only one exporter.

Things to consider:

• How to handle different targets/credentials/settings in the config file
• Environment variables may no longer be viable if there is more than one target

[scaleoutsean]

Environment variables may no longer be viable if there is more than one target

We could use shell variables (e.g. without exporting them) to run multiple instances of solidfire-exporter. I tried with screen it works fine. More in this comment.