implement running and total mnesia node count metrics

Author: luos

src/collectors/mnesia/prometheus_mnesia_collector.erl

@@ -157,7 +157,13 @@ metrics(EnabledMetrics) ->
     fun() -> TablewiseMemoryUsage end},
    {tablewise_size, gauge,
     "Number of rows present per table",
-    fun() -> TablewiseSize end}
+    fun() -> TablewiseSize end},
+    {known_node_count, gauge,
+    "Count of known mnesia nodes",
+    fun() -> length(mnesia:system_info(db_nodes)) end},
+    {running_node_count, gauge,
+    "Count of running mnesia nodes",
+    fun() -> length(mnesia:system_info(running_db_nodes)) end}
     ].
 
 %%====================================================================

test/eunit/collectors/mnesia/prometheus_mnesia_collector_tests.erl

@@ -53,6 +53,8 @@ test_mnesia_on_collector_env_off() ->
   ?assertMatch(nomatch, re:run(Metrics, "erlang_mnesia_logged_transactions")),
   ?assertMatch(nomatch, re:run(Metrics, "erlang_mnesia_restarted_transactions")),
   ?assertMatch(nomatch, re:run(Metrics, "erlang_mnesia_memory_usage_bytes")),
+  ?assertMatch(nomatch, re:run(Metrics, "erlang_mnesia_total_node_count")),
+  ?assertMatch(nomatch, re:run(Metrics, "erlang_mnesia_running_node_count")),
   application:unset_env(prometheus,mnesia_collector_metrics,[]).
 
 test_mnesia_on_collector() ->
@@ -66,7 +68,9 @@ test_mnesia_on_collector() ->
   ?assertMatch({match,_}, re:run(Metrics, "erlang_mnesia_committed_transactions")),
   ?assertMatch({match,_}, re:run(Metrics, "erlang_mnesia_logged_transactions")),
   ?assertMatch({match,_}, re:run(Metrics, "erlang_mnesia_restarted_transactions")),
-  ?assertMatch({match,_}, re:run(Metrics, "erlang_mnesia_memory_usage_bytes")).
+  ?assertMatch({match,_}, re:run(Metrics, "erlang_mnesia_memory_usage_bytes")),
+  ?assertMatch({match,_}, re:run(Metrics, "erlang_mnesia_known_node_count 1")),
+  ?assertMatch({match,_}, re:run(Metrics, "erlang_mnesia_running_node_count 1")).
 
 test_mnesia_on_collector_global_labels() ->
   Metrics = try