fix(thumbnails): Implement ratelimit for the grpc service

This moves the ratelimit ('THUMBNAILS_MAX_CONCURRENT_REQUESTS') from the
HTTP endpoint to the GRPC endpoint. The HTTP endpoint is just used for
downloading already created thumbnails. But the resource consuming part of
thumbnail generation happens in the GRPC service.

services/thumbnails/pkg/command/server.go

@@ -59,6 +59,7 @@ func Server(cfg *config.Config) *cli.Command {
 				grpc.Address(cfg.GRPC.Addr),
 				grpc.Metrics(m),
 				grpc.TraceProvider(traceProvider),
+				grpc.MaxConcurrentRequests(cfg.GRPC.MaxConcurrentRequests),
 			)
 
 			gr.Add(service.Run, func(_ error) {
@@ -91,7 +92,6 @@ func Server(cfg *config.Config) *cli.Command {
 				http.Metrics(m),
 				http.Namespace(cfg.HTTP.Namespace),
 				http.TraceProvider(traceProvider),
-				http.MaxConcurrentRequests(cfg.HTTP.MaxConcurrentRequests),
 			)
 			if err != nil {
 				logger.Info().