Add toggle to deploy db exporters for external databases

Feature request description

Customer 1615 is requesting a toggle in our Helm charts, to deploy the database exporter containers, even when using off-cluster managed databases (ex. AWS RDS), so they can still get app-specific database metrics in the Sourcegraph-provided Grafana dashboards.

Is your feature request related to a problem? If so, please describe.

Sourcegraph's Grafana dashboards are much more useful while monitoring performance and troubleshooting issues, when database metrics are shown, so that customers don't need to configure an external metrics system (ex. AWS CloudWatch Metrics) to get the same metrics.

Describe alternatives you've considered.

Configuring an external metrics system.

Additional context

Slack thread in #discuss-release-ship