Add v26.3 API auth section and critical nodes request instructions

Author: rmloveland

src/current/v26.3/monitoring-and-alerting.md

@@ -79,6 +79,25 @@ If the cluster is unavailable, a database's `crdb_internal` system catalog canno
 
 For details, see [`crdb_internal`]({% link {{ page.version.version }}/crdb-internal.md %}).
 
+### Authenticate to API endpoints
+
+To call the HTTP API endpoints on this page using `curl`:
+
+For an insecure or local testing cluster, use HTTP:
+
+{% include_cached copy-clipboard.html %}
+~~~ shell
+curl http://<host>:<http-port>/<endpoint>
+~~~
+
+For a secure cluster, authenticate to the HTTPS endpoint with [`cockroach auth-session login`]({% link {{ page.version.version }}/cockroach-auth-session.md %}#log-in-to-the-http-interface), then pass the authentication cookie to `curl`.
+
+{% include_cached copy-clipboard.html %}
+~~~ shell
+cockroach auth-session login <user> --certs-dir=certs --only-cookie > $HOME/.cockroachdb_api_key
+curl --cookie $HOME/.cockroachdb_api_key --cacert certs/ca.crt https://<host>:<http-port>/<endpoint>
+~~~
+
 ### Health endpoints
 
 CockroachDB provides two HTTP endpoints for checking the health of individual nodes.
@@ -191,6 +210,10 @@ The critical nodes status endpoint is used to:
 
 If you find under-replicated ranges or constraint violations, you will need to [Troubleshoot your replication zones]({% link {{ page.version.version }}/troubleshoot-replication-zones.md %}).
 
+#### Request the endpoint
+
+To return the JSON response, send a `POST` request to `/_status/critical_nodes`. For authentication details, refer to [Authenticate to API endpoints](#authenticate-to-api-endpoints).
+
 #### Fields
 
 The JSON object returned by the critical nodes status endpoint contains the following top-level fields.