Expose number of unreachable cluster peers via Prometheus

Unreachable peers is a subset of DB cluster nodes that are not connected
to the current node via Erlang distribution for any reason.

(cherry picked from commit e009a0a)

Author: gomoripeti

deps/rabbitmq_prometheus/app.bzl

@@ -12,6 +12,7 @@ def all_beam_files(name = "all_beam_files"):
             "src/collectors/prometheus_process_collector.erl",
             "src/collectors/prometheus_rabbitmq_alarm_metrics_collector.erl",
             "src/collectors/prometheus_rabbitmq_core_metrics_collector.erl",
+            "src/collectors/prometheus_rabbitmq_dynamic_collector.erl",
             "src/collectors/prometheus_rabbitmq_global_metrics_collector.erl",
             "src/rabbit_prometheus_app.erl",
             "src/rabbit_prometheus_dispatcher.erl",
@@ -42,6 +43,7 @@ def all_test_beam_files(name = "all_test_beam_files"):
             "src/collectors/prometheus_process_collector.erl",
             "src/collectors/prometheus_rabbitmq_alarm_metrics_collector.erl",
             "src/collectors/prometheus_rabbitmq_core_metrics_collector.erl",
+            "src/collectors/prometheus_rabbitmq_dynamic_collector.erl",
             "src/collectors/prometheus_rabbitmq_global_metrics_collector.erl",
             "src/rabbit_prometheus_app.erl",
             "src/rabbit_prometheus_dispatcher.erl",
@@ -83,6 +85,7 @@ def all_srcs(name = "all_srcs"):
             "src/collectors/prometheus_process_collector.erl",
             "src/collectors/prometheus_rabbitmq_alarm_metrics_collector.erl",
             "src/collectors/prometheus_rabbitmq_core_metrics_collector.erl",
+            "src/collectors/prometheus_rabbitmq_dynamic_collector.erl",
             "src/collectors/prometheus_rabbitmq_global_metrics_collector.erl",
             "src/rabbit_prometheus_app.erl",
             "src/rabbit_prometheus_dispatcher.erl",

deps/rabbitmq_prometheus/src/collectors/prometheus_rabbitmq_dynamic_collector.erl

@@ -0,0 +1,42 @@
+%%% Collector for dynamic metrics that are calculated at collection time
+-module(prometheus_rabbitmq_dynamic_collector).
+
+-behaviour(prometheus_collector).
+-include_lib("prometheus/include/prometheus.hrl").
+
+-export([deregister_cleanup/1,
+         collect_mf/2]).
+
+-define(METRIC_NAME_PREFIX, "rabbitmq_").
+
+-define(METRICS, [{unreachable_cluster_peers_count, gauge,
+                   "Number of peers in the cluster the current node cannot reach."}
+                 ]).
+
+%%====================================================================
+%% Collector API
+%%====================================================================
+
+deregister_cleanup(_) -> ok.
+
+collect_mf(_Registry, Callback) ->
+    _ = lists:foreach(
+          fun({Name, Type, Help}) ->
+                  Callback(
+                    prometheus_model_helpers:create_mf(
+                      ?METRIC_NAME(Name),
+                      Help,
+                      Type,
+                      values(Name))
+                   )
+          end,
+          ?METRICS
+         ),
+    ok.
+
+%%====================================================================
+%% Private Parts
+%%====================================================================
+
+values(unreachable_cluster_peers_count) ->
+    [{[], length(rabbit_nodes:list_unreachable())}].

deps/rabbitmq_prometheus/src/rabbit_prometheus_dispatcher.erl

@@ -17,6 +17,7 @@ build_dispatcher() ->
         prometheus_rabbitmq_core_metrics_collector,
         prometheus_rabbitmq_global_metrics_collector,
         prometheus_rabbitmq_alarm_metrics_collector,
+        prometheus_rabbitmq_dynamic_collector,
         prometheus_process_collector]),
     prometheus_registry:register_collectors('per-object', [
         prometheus_vm_system_info_collector,