Add TCM cluster metrics doc (#4273)

Resolves #4248

Co-authored-by: Andrey Aksenov <[email protected]>

Author: p7nov

doc/reference/tooling/tcm/index.rst

@@ -39,6 +39,7 @@ to read data. LDAP authorization is supported as well.
     tcm_connect_clusters
     tcm_cluster_config
     tcm_cluster_monitoring
+    tcm_cluster_metrics
     tcm_access_control
     tcm_audit_log
     tcm_configuration

doc/reference/tooling/tcm/tcm_cluster_metrics.rst

@@ -0,0 +1,50 @@
+..  _tcm_cluster_metrics:
+
+Viewing cluster metrics
+=======================
+
+..  include:: index.rst
+    :start-after: ee_note_tcm_start
+    :end-before: ee_note_tcm_end
+
+In |tcm_full_name|, you can view metrics of connected clusters in real time on the
+**Cluster** > **Cluster metrics** page. The list of metrics that Tarantool exposes
+is provided in the :ref:`metrics-reference`.
+
+Metrics are displayed one by one. To view a metric, select it in the drop-down list
+at the top of the page. Then, choose a way to visualize it:
+
+- **Chart**: a time series chart with the metric values displayed as lines.
+- **Table**: a table where the metric values are displayed as numbers in table cells.
+
+Once you select a metric, |tcm| starts visualizing its current values, updating them
+once per second. To pause the visualization, click the button on the left from
+the metrics selector. To stop the visualization, clear the metric selection.
+
+
+..  _tcm_cluster_metrics_prometheus:
+
+Monitoring metrics with Prometheus
+----------------------------------
+
+To allow collecting cluster metrics with external systems, such as Prometheus,
+|tcm| provides HTTP endpoints at ``/api/metrics/<clusterId>``.
+
+.. note::
+
+    Cluster IDs are shown in the cluster selection dialog that opens when you click
+    **Cluster** at the top of the left navigation pane.
+
+To access such an endpoint, a request must be authorized with an :ref:`API token <tcm_api_tokens>`
+that has a ``cluster.metrics`` permission on the target cluster.
+
+Below is an example of a Prometheus scrape configuration that collects metrics of
+a Tarantool cluster from |tcm|:
+
+.. code-block:: yaml
+
+    - job_name: "tarantool"
+        static_configs:
+          - targets: ["127.0.0.1:8080"]
+        metrics_path: "/api/metrics/00000000-0000-0000-0000-000000000000"
+        bearer_token: QgMPZ22JZ3uw7n0QTbqYGAQDmNDs1JnTkhaC1OlQzWM3utmpV78b23GG97zp8YE3