Update otel-collector-config configmap (#4199)

The otel-collector-config configmap referenced in
https://knative.dev/docs/admin/collecting-metrics/#set-up-the-collector
has incorrect configuration which leads to a config loading issue causing
the otel-collector to go into a crashloop. This PR updates the config map
as per https://opentelemetry.io/docs/collector/configuration/

docs/admin/collecting-metrics/collector.yaml

@@ -13,12 +13,12 @@ data:
       logging:
       prometheus:
         endpoint: "0.0.0.0:8889"
-
-    service:
-      extensions:
+    extensions:
         health_check:
         pprof:
-        zpages:
+        zpages
+    service:
+      extensions: [health_check, pprof, zpages]
       pipelines:
         metrics:
           receivers: [opencensus]