fix: prometheus metrics annotations

## Why

Previously, the `_node/_local/_prometheus`
endpoints would return invalid annotations
for certain metrics. For example:

```
# TYPE couchdb_database_reads_total counterof times a document was read from a database
couchdb_database_reads_total 2208
```

In general this doesn't matter too much - prometheus
will infer the type from the metric name - but
some clients will respect it and generate
errors/warnings if the annotations are incorrect.

## What

This removes the carriage returns from the
prometheus output. These were added as an incorrect
fix to spurious newlines in the output but
caused this bug with the annotations.

The actual bug causing the additional newlines is
that the result of `type_def/3` was additionally
wrapped in a list.

This commit removes the re-wrapping. removes the
carriage return and adds a test for the
output. This changes the output of `to_prom` slightly
because the first element of the result is no longer

Author: willholley

src/couch_prometheus/src/couch_prometheus_util.erl

@@ -116,7 +116,7 @@ couch_to_prom(Path, Info, _All) ->
 type_def(Metric, Type, Desc) ->
     Name = to_prom_name(Metric),
     [
-        to_bin(io_lib:format("\n# HELP ~s ~s\r", [Name, Desc])),
+        to_bin(io_lib:format("\n# HELP ~s ~s", [Name, Desc])),
         to_bin(io_lib:format("# TYPE ~s ~s", [Name, Type]))
     ].
 
@@ -126,7 +126,7 @@ to_prom(_Metric, _Type, _Desc, []) ->
     [];
 to_prom(Metric, Type, Desc, Instances) when is_list(Instances) ->
     TypeStr = type_def(Metric, Type, Desc),
-    [TypeStr] ++ lists:flatmap(fun(Inst) -> to_prom(Metric, Inst) end, Instances);
+    TypeStr ++ lists:flatmap(fun(Inst) -> to_prom(Metric, Inst) end, Instances);
 to_prom(Metric, Type, Desc, Data) ->
     to_prom(Metric, Type, Desc, [Data]).
 
@@ -221,6 +221,23 @@ desc(Info) ->
 -ifdef(TEST).
 -include_lib("couch/include/couch_eunit.hrl").
 
+to_prom_annotations_test() ->
+    Out = to_prom(couchdb_bt_engine_cache_size, gauge, "number of entries in the btree cache", 9),
+    [
+        ?assertEqual(
+            <<"\n# HELP couchdb_couchdb_bt_engine_cache_size number of entries in the btree cache">>,
+            lists:nth(1, Out)
+        ),
+        ?assertEqual(
+            <<"# TYPE couchdb_couchdb_bt_engine_cache_size gauge">>,
+            lists:nth(2, Out)
+        ),
+        ?assertEqual(
+            <<"couchdb_couchdb_bt_engine_cache_size 9">>,
+            lists:nth(3, Out)
+        )
+    ].
+
 to_prom_counter_test() ->
     [
         ?assertEqual(
@@ -288,10 +305,10 @@ counter_metric_test_() ->
 
 test_to_prom_output(Metric, Type, Desc, Val) ->
     Out = to_prom(Metric, Type, Desc, Val),
-    lists:nth(2, Out).
+    lists:nth(3, Out).
 
 test_to_prom_summary_output(Metric, Info) ->
     Out = to_prom_summary(Metric, Info),
-    lists:nth(3, Out).
+    lists:nth(4, Out).
 
 -endif.

src/couch_prometheus/test/eunit/couch_prometheus_e2e_tests.erl

@@ -177,12 +177,12 @@ t_survives_mem3_sync_termination(_) ->
     ok = supervisor:terminate_child(mem3_sup, mem3_sync),
     ?assertEqual(undefined, whereis(mem3_sync)),
     ?assertMatch(
-        [[_, _], <<"couchdb_internal_replication_jobs 0">>],
+        [_, _, <<"couchdb_internal_replication_jobs 0">>],
         couch_prometheus:get_internal_replication_jobs_stat()
     ),
     {ok, _} = supervisor:restart_child(mem3_sup, mem3_sync),
     ?assertMatch(
-        [[_, _], <<"couchdb_internal_replication_jobs", _/binary>>],
+        [_, _, <<"couchdb_internal_replication_jobs", _/binary>>],
         couch_prometheus:get_internal_replication_jobs_stat()
     ).