fix: prometheus counter metric naming (#4474)

`couch_prometheus_util:counter_metric/1` is intended to add a
`_total` suffix to the name of metrics with type `counter` if
the name does not already end with `_total`.

The implementation was previously incorrect, resulting in metrics
with names like `document_purges_total_total`.

This adds basic eunit tests for the failure / success scenarios and
fixes the implementation.

It is a breaking change for consumers of the `_node/_local/_prometheus`
endpoint, however, since the following metric is renamed:

* `couchdb_document_purges_total_total` -> `couchdb_document_purges_total`

src/couch_prometheus/src/couch_prometheus_util.erl

@@ -146,9 +146,9 @@ path_to_name(Path) ->
 
 counter_metric(Path) ->
     Name = path_to_name(Path),
-    case string:find(Name, <<"_total">>, trailing) == <<"_total">> of
-        true -> Name;
-        false -> to_bin(io_lib:format("~s_total", [Name]))
+    case lists:suffix("_total", Name) of
+        true -> to_bin(Name);
+        _ -> to_bin(io_lib:format("~s_total", [Name]))
     end.
 
 to_bin(Data) when is_list(Data) ->
@@ -167,3 +167,77 @@ val(Data) ->
 val(Key, Stats) ->
     {Key, Data} = lists:keyfind(Key, 1, Stats),
     val(Data).
+
+-ifdef(TEST).
+-include_lib("couch/include/couch_eunit.hrl").
+
+to_prom_counter_test() ->
+    ?assertEqual(
+        <<"couchdb_ddoc_cache 10">>,
+        test_to_prom_output(ddoc_cache, counter, 10)
+    ),
+    ?assertEqual(
+        <<"couchdb_httpd_status_codes{code=\"200\"} 3">>,
+        test_to_prom_output(httpd_status_codes, counter, {[{code, 200}], 3})
+    ).
+
+to_prom_gauge_test() ->
+    ?assertEqual(
+        <<"couchdb_temperature_celsius 36">>,
+        test_to_prom_output(temperature_celsius, gauge, 36)
+    ).
+
+to_prom_summary_test() ->
+    ?assertEqual(
+        <<"couchdb_mango_query_time_seconds{quantile=\"0.75\"} 4.5">>,
+        test_to_prom_summary_output([mango_query_time], [
+            {value, [
+                {min, 0.0},
+                {max, 0.0},
+                {arithmetic_mean, 0.0},
+                {geometric_mean, 0.0},
+                {harmonic_mean, 0.0},
+                {median, 0.0},
+                {variance, 0.0},
+                {standard_deviation, 0.0},
+                {skewness, 0.0},
+                {kurtosis, 0.0},
+                {percentile, [
+                    {50, 0.0},
+                    {75, 4500},
+                    {90, 0.0},
+                    {95, 0.0},
+                    {99, 0.0},
+                    {999, 0.0}
+                ]},
+                {histogram, [
+                    {0, 0}
+                ]},
+                {n, 0}
+            ]},
+            {type, histogram},
+            {desc, <<"length of time processing a mango query">>}
+        ])
+    ).
+
+counter_metric_test_() ->
+    [
+        ?_assertEqual(
+            <<"document_purges_total">>,
+            counter_metric([document_purges, total])
+        ),
+        ?_assertEqual(
+            <<"document_purges_total">>,
+            counter_metric([document_purges])
+        )
+    ].
+
+test_to_prom_output(Metric, Type, Val) ->
+    Out = to_prom(Metric, Type, Val),
+    lists:nth(2, Out).
+
+test_to_prom_summary_output(Metric, Info) ->
+    Out = to_prom_summary(Metric, Info),
+    lists:nth(3, Out).
+
+-endif.