rabbit_prometheus_handler: Delete manual gzip handling

The Cowboy stream handler `cowboy_compress_h` is already enabled for
this endpoint so there's no need to perform gzipping manually.

Author: the-mikedavis

deps/rabbitmq_prometheus/src/rabbit_prometheus_handler.erl

@@ -59,7 +59,7 @@ setup_metrics(Registry) ->
                   {labels, ["registry", "content_type"]},
                   {registry, Registry}],
     ScrapeEncodedSize = [{name, ?SCRAPE_ENCODED_SIZE},
-                         {help, "Scrape size, encoded"},
+                         {help, "Scrape size, encoded (deprecated)"},
                          {labels, ["registry", "content_type", "encoding"]},
                          {registry, Registry}],
 
@@ -104,43 +104,31 @@ format_metrics(Request, Registry) ->
     ContentType = prometheus_text_format:content_type(),
     Encoding = accept_encoding_header:negotiate(AcceptEncoding, [<<"identity">>,
                                                                  <<"gzip">>]),
-    Headers = #{<<"content-type">> => ContentType,
-                <<"content-encoding">> => Encoding},
-    case Encoding of
-        <<"gzip">> ->
-            Scrape = prometheus_summary:observe_duration(
-                       Registry,
-                       ?SCRAPE_DURATION,
-                       [Registry, ContentType],
-                       fun () -> prometheus_text_format:format(Registry) end),
-            prometheus_summary:observe(Registry,
-                                       ?SCRAPE_SIZE,
-                                       [Registry, ContentType],
-                                       iolist_size(Scrape)),
-            Encoded = zlib:gzip(Scrape),
-            prometheus_summary:observe(telemetry_registry(),
-                                       ?SCRAPE_ENCODED_SIZE,
-                                       [Registry, ContentType, Encoding],
-                                       iolist_size(Encoded)),
-            cowboy_req:reply(200, Headers, Encoded, Request);
-        <<"identity">> ->
-            Req = cowboy_req:stream_reply(200, Headers, Request),
-            Fmt = fun(Data, Size) ->
-                cowboy_req:stream_body(Data, nofin, Req),
-                Size + iolist_size(Data)
-            end,
-            Size = prometheus_summary:observe_duration(
-                     Registry,
-                     ?SCRAPE_DURATION,
-                     [Registry, ContentType],
-                     fun () -> prometheus_text_format:format_into(Registry, 0, Fmt) end),
-            cowboy_req:stream_body(<<>>, fin, Req),
-            prometheus_summary:observe(Registry,
-                                       ?SCRAPE_SIZE,
-                                       [Registry, ContentType],
-                                       Size),
-            Req
-    end.
+    Headers0 = #{<<"content-type">> => ContentType},
+    Headers = case Encoding of
+                  <<"gzip">> ->
+                      %% `cowboy_compress_h` transparently compresses the data
+                      %% while streaming if `content-encoding` is unset.
+                      Headers0;
+                  _ ->
+                      Headers0#{<<"content-encoding">> => Encoding}
+              end,
+    Req = cowboy_req:stream_reply(200, Headers, Request),
+    Fmt = fun(Data, Size) ->
+        cowboy_req:stream_body(Data, nofin, Req),
+        Size + iolist_size(Data)
+    end,
+    Size = prometheus_summary:observe_duration(
+             Registry,
+             ?SCRAPE_DURATION,
+             [Registry, ContentType],
+             fun () -> prometheus_text_format:format_into(Registry, 0, Fmt) end),
+    cowboy_req:stream_body(<<>>, fin, Req),
+    prometheus_summary:observe(Registry,
+                               ?SCRAPE_SIZE,
+                               [Registry, ContentType],
+                               Size),
+    Req.
 
 validate_registry(undefined, auto) ->
     {true, default};