Have a custom prefix for each of the default metrics created by postgres_exporter

[ajaybhat]

Currently postgres_exporter exports default metrics of the form pg_metric_column. For eg. pg_stat_replication_pid. Since we may write additional queries to get more metrics, through the PG_EXPORTER_EXTEND_QUERY_PATH, we would have customised metric names in the queries.yaml file, for eg foo_bar_replication. It would be nice to have a way to have the existing default metrics also have the foo_bar prefix, so we can collect all the metrics under single logical umbrella.

[ajaybhat]

I could work on this, if needed

[uspen]

I use --disable-default-metrics and write all my metrics to extended queries