Prune fields with null values in user-provided Alertmanager config

This allows us to gradually roll out Alertmanager configs with new
fields set (e.g. `opsgenie_configs.source` in a receiver config) even
though the receiver config is formatted as a list which isn't easily
editable in the hierarchy.

component/alert-routing-discovery.libsonnet

@@ -73,7 +73,7 @@ local teamBasedRouting = std.map(
 
 local alertmanagerConfig =
   local routes = std.get(params.openshift4_monitoring.alertManagerConfig.route, 'routes', []);
-  params.openshift4_monitoring.alertManagerConfig {
+  std.prune(params.openshift4_monitoring.alertManagerConfig) {
     receivers+: [ { name: nullReceiver } ],
     route+: {
       routes: adParams.prepend_routes + teamBasedRouting + adParams.append_routes + routes + if ownerOrFallbackTeam != null then [ {

docs/modules/ROOT/pages/references/parameters.adoc

@@ -268,6 +268,8 @@ A dictionary holding the configuration for the AlertManager.
 
 See the https://docs.openshift.com/container-platform/latest/monitoring/managing-alerts.html#applying-custom-alertmanager-configuration_managing-alerts[OpenShift docs] for available parameters.
 
+The component will silently drop any fields in the provided config which are empty.
+The component treats `null` as empty for scalar fields.
 
 == `alertManagerAutoDiscovery`
 

tests/golden/team-routing/openshift4-monitoring/openshift4-monitoring/10_alertmanager_config.yaml

@@ -25,6 +25,7 @@ stringData:
       "target_match_re":
         "severity": "info"
     "receivers":
+    - "name": "foo"
     - "name": "__component_openshift4_monitoring_null"
     "route":
       "group_interval": "5s"

tests/golden/team-routing/openshift4-monitoring/openshift4-monitoring/99_discovery_debug_cm.yaml

@@ -17,6 +17,7 @@ data:
       "target_match_re":
         "severity": "info"
     "receivers":
+    - "name": "foo"
     - "name": "__component_openshift4_monitoring_null"
     "route":
       "group_interval": "5s"

tests/team-routing.yml

@@ -38,6 +38,10 @@ parameters:
 
   openshift4_monitoring:
     alertManagerConfig:
+      receivers:
+        - name: foo
+          opsgenie_configs:
+            - source: null
       route:
         routes:
           - receiver: other