support empty list boundaries param to get a single bucket histogram

Author: tsloughter

apps/opentelemetry_experimental/src/otel_meter_default.erl

@@ -97,6 +97,9 @@ validate_advisory_param(Name, _Kind, Opt, _Value) ->
     ?LOG_WARNING("[instrument '~s'] '~s' advisory parameter is not supported, ignoring", [Name, Opt]),
     false.
 
+%% empty list denotes a single bucket histogram that can be used for things like summaries
+validate_explicit_bucket_boundaries(_Name, []) ->
+    {true, {explicit_bucket_boundaries, []}};
 validate_explicit_bucket_boundaries(Name, [_ | _] = Value) ->
     case lists:all(fun is_number/1, Value) and (lists:sort(Value) == Value) of
         true ->

apps/opentelemetry_experimental/test/otel_metrics_SUITE.erl

@@ -1382,6 +1382,11 @@ advisory_params(_Config) ->
                                   #{advisory_params => #{explicit_bucket_boundaries => [10, 20, 30]}}),
     ?assertEqual(Histogram#instrument.advisory_params, #{explicit_bucket_boundaries => [10, 20, 30]}),
 
+    %% an empty boundaries list can be used to get a single bucket histogram `(-Inf, +Inf)'
+    BHistogram = otel_histogram:create(Meter, b_histogram,
+                                       #{advisory_params => #{explicit_bucket_boundaries => []}}),
+    ?assertEqual(BHistogram#instrument.advisory_params, #{explicit_bucket_boundaries => []}),
+
     Ctx = otel_ctx:new(),
 
     ?assertEqual(ok, otel_histogram:record(Ctx, Histogram, 15, #{<<"a">> => <<"1">>})),