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.json

@@ -421,6 +421,8 @@
     }
   ],
   "reporting": {
+    "address": "http://localhost:8080",
+    "export_interval": "15m",
     "license": {
       "enabled": "true"
     }