databases/prometheus-postgres-exporter: Allow environment to be set

[ivy]

Summary

When not using multi-target mode, postgres_exporter relies on its
environment for configuration of the monitoring target. Add a new
postgres_exporter_env rc.conf option to allow this to be configured.

PR: 290694

[ivy]

would you approve an MFH for this change? it's pretty straightforward and provides a useful feature.

[arrowd]

Sure.

[ivy]

thanks.