[bugfix] Let prometheus client do its own compression handling (#3232)

cmd/gotosocial/action/server/server.go

@@ -439,13 +439,18 @@ var Start action.GTSAction = func(ctx context.Context) error {
 	fsThrottle := middleware.Throttle(cpuMultiplier, retryAfter) // fileserver / web templates / emojis
 	pkThrottle := middleware.Throttle(cpuMultiplier, retryAfter) // throttle public key endpoint separately
 
-	gzip := middleware.Gzip() // applied to all except fileserver
+	// Gzip middleware is applied to all endpoints except
+	// fileserver (compression too expensive for those),
+	// health (which really doesn't need compression), and
+	// metrics (which does its own compression handling that
+	// is rather annoying to neatly override).
+	gzip := middleware.Gzip()
 
 	// these should be routed in order;
 	// apply throttling *after* rate limiting
 	authModule.Route(route, clLimit, clThrottle, gzip)
 	clientModule.Route(route, clLimit, clThrottle, gzip)
-	metricsModule.Route(route, clLimit, clThrottle, gzip)
+	metricsModule.Route(route, clLimit, clThrottle)
 	healthModule.Route(route, clLimit, clThrottle)
 	fileserverModule.Route(route, fsMainLimit, fsThrottle)
 	fileserverModule.RouteEmojis(route, instanceAccount.ID, fsEmojiLimit, fsThrottle)

internal/api/metrics/metrics.go

@@ -30,9 +30,13 @@ type Module struct {
 }
 
 func New() *Module {
-	// Use our own gzip handler.
+	// Let prometheus use "identity", ie., no compression,
+	// or "gzip", to match our own gzip compression middleware.
 	opts := promhttp.HandlerOpts{
-		DisableCompression: true,
+		OfferedCompressions: []promhttp.Compression{
+			promhttp.Identity,
+			promhttp.Gzip,
+		},
 	}
 
 	// Instrument handler itself.

testrig/config.go

@@ -154,7 +154,7 @@ func testDefaults() config.Configuration {
 		TracingTransport:         "grpc",
 		TracingInsecureTransport: true,
 
-		MetricsEnabled:     false,
+		MetricsEnabled:     true,
 		MetricsAuthEnabled: false,
 
 		SyslogEnabled:  false,