Fix proto serializer (#8147)

Author: jack-berg

exporters/common/src/main/java/io/opentelemetry/exporter/internal/marshal/ProtoSerializer.java

@@ -277,9 +277,9 @@ public void writeSerializedMessage(byte[] protoSerialized, String jsonSerialized
 
   @Override
   public void close() throws IOException {
+    idCache.clear();
     try {
       output.flush();
-      idCache.clear();
     } catch (IOException e) {
       // If close is called automatically as part of try-with-resources, it's possible that
       // output.flush() will throw the same exception. Re-throwing the same exception in a finally