reporting: allow export interval and address to be configurable (#23674)

The go-census library supports configuration to send metrics to a local
development version of the collector. Add "undocumented" configuration options
to the `reporting` block allow developers to debug and verify we're sending the
data we expect with real Nomad servers and not just unit tests.

Ref: https://hashicorp.atlassian.net/browse/NET-10057
Ref: https://github.com/hashicorp/nomad-enterprise/pull/1708

command/agent/testdata/basic.hcl

@@ -348,6 +348,9 @@ reporting {
   license {
     enabled = true
   }
+
+  address         = "http://localhost:8080"
+  export_interval = "15m"
 }
 
 keyring "awskms" {