Allow application to pre-render labels for text format

Builds on top of prometheus-erl#136

This is useful when e.g. escaping is not needed, as it is in case with
RabbitMQ, where no escaping is needed because strict rules already
apply to names.

This change provides not only direct speedup, but also significantly
reduced garbage collection overhead, as for every output line the
proplist with labels and the list with converted `#'LabelPair'{}`'s
are replaced with just a single binary.

Using RabbitMQ as an example, time to produce ~630k metrics went from
~6 to ~3 seconds.

Downside is that it's only compatible with text format, and spec for
`#'LabelPair'{}' is not respected.

Author: binarin

src/model/prometheus_model_helpers.erl

@@ -50,7 +50,8 @@
 -type label_name() :: term().
 -type label_value() :: term().
 -type label() :: {label_name(), label_value()}.
--type labels() :: [label()].
+-type pre_rendered_labels() :: binary().
+-type labels() :: [label()] | pre_rendered_labels().
 -type value() :: float() | integer() | undefined | infinity.
 -type prometheus_boolean() :: boolean() | number() | list() | undefined.
 -type gauge() :: value() | {value()} | {labels(), value()}.
@@ -328,6 +329,18 @@ histogram_metric(Labels, Buckets, Count, Sum) ->
 %% @doc Equivalent to
 %% {@link label_pair/1. `lists:map(fun label_pair/1, Labels)'}.
 %% @end
+%%
+%% NB `is_binary' clause here is for a special optimization for text
+%% format only: client code can pre-generate final labels string,
+%% e.g. when it knows when character escaping is not needed. This
+%% avoids direct performance cost of character escaping, and also
+%% reduces garabage collection pressure, as intermediate lists of
+%% tuples/records are not created at all. This optimization is used by
+%% RabbitMQ prometheus plugin (which calls `create_mf/5', and it ends
+%% here).
+%% WARNING Works only for text format, protobuf format export will
+%% fail with an error.
+label_pairs(B) when is_binary(B) -> B;
 label_pairs(Labels) -> lists:map(fun label_pair/1, Labels).
 
 %% @doc