Support generic addon to allow specify metric query in configuration

[maxwangvmt]

Currently, when added one addon to support a new exporter, we need to write code for the exporter.

After looking into some examples as Cassandra and XL server, it seems that we can have one generic addon that can allow users to specify the queries to get the metrics and the mapping from the metrics to the corresponding commodities. For example, user can simply add following to a configuration file:

promethuest-query-1 RESPONSE_TIME
promethuest-query-2 TRANSACTION

and prometurbo will take the settings and query the promethues server for the metrics and generate response time and transaction commodities as in our current prometurbo mechanism.

The above is a simplified version but as discussed with the team, this generic addon should be doable and shouldn't be too complicated. Need further investigation and think an optimal way for the framework.