Allow custom service names for exporters

For the repeatable db provisioning project we are using the following Prometheus exporters:

• node-exporter
• postgresql-exporter

(We are not currently using Omnibus pgbouncer because of #4796 (closed))

For at least these two exporters, we would like to be able to configure custom service names/tags when they are registered in Consul. This will allow us to have a single Consul server, servicing multiple database shards.

Note that for Patroni, which also registeres itself with Consul, we can set patroni['scope'] which will set the service name.

For example, if we have two shards CI and main:

• node-exporter-main
• node-exporter-ci
• postgresql-exporter-main
• postgresql-exporter-ci
• postgresql-main [master/replica]
• postgresql-ci [master/replica]