agent: allow configuration of in-memory telemetry sink. (#20166)

This change adds configuration options for setting the in-memory
telemetry sink collection and retention durations. This sink backs
the metrics JSON API and previously had hard-coded default values.

The new options are particularly useful when running development or
debug environments, where metrics collection is desired at a fast
and granular rate.

command/agent/testdata/basic.json

@@ -170,7 +170,6 @@
       "server_service_name": "nomad",
       "service_auth_method": "nomad-services",
       "task_auth_method": "nomad-tasks",
-
       "service_identity": {
         "aud": [
           "consul.io",
@@ -361,6 +360,8 @@
   "syslog_facility": "LOCAL1",
   "telemetry": [
     {
+      "in_memory_collection_interval": "1m",
+      "in_memory_retention_period": "24h",
       "collection_interval": "3s",
       "disable_hostname": true,
       "prometheus_metrics": true,
@@ -394,7 +395,10 @@
       "cert_file": "/path/to/cert/file",
       "create_from_role": "test_role",
       "default_identity": {
-        "aud": ["vault.io", "nomad.io"],
+        "aud": [
+          "vault.io",
+          "nomad.io"
+        ],
         "env": false,
         "file": true,
         "ttl": "3h"
@@ -408,9 +412,9 @@
       "token": "12345"
     }
   ],
-  "reporting":{
-    "license":{
-      "enabled":"true"
+  "reporting": {
+    "license": {
+      "enabled": "true"
     }
   }
 }