Use begin-end block with at-lock macro to help coverage

src/Prometheus.jl

@@ -1042,9 +1042,11 @@ end
 function expose_io(io::IO, reg::CollectorRegistry)
     # Collect all metrics
     metrics = Metric[]
-    @lock reg.lock for collector in reg.collectors
+    @lock reg.lock begin
+        for collector in reg.collectors
             collect!(metrics, collector)
         end
+    end
     sort!(metrics; by = metric -> metric.metric_name)
     # Write to IO
     buf = IOBuffer(; maxsize=1024^2) # 1 MB