Feature request: Support metrics from DataDog-style stored procedures

[mohag]

DataDog support collecting metrics by calling stored procedures (at least on MS SQL Server databases). They return metric, type, value and tag columns. (It seems like that is mainly limited to SQL server databases, other databases has other methods for custom metrics...)

They are directly usable if all of them returns the same metric, but cases with multiple metrics in the output require that the output is filtered and that the stored procedure is run multiple times.

Support for this would make migration from DataDog to Prometheus simpler.

(It also supports a different format for PostgreSQL custom metrics) (This is closer to what is currently supported)