根据 Android Studio 3.1发行说明：

可从命令行使用的设备监视器

在Android Studio 3.1中，设备监视器提供的服务较少角色比以前更重要。在许多情况下，新的和经过改进的工具现在可以通过设备监视器提供的功能。

请参见设备监视器文档，以获取有关从命令行调用设备监视器的说明以及通过设备监视器可用的工具的详细信息。

因此，如果仍然要运行设备监视器，则需要从命令行手动启动它。否则，请迁移到更新的性能工具和更新了直接在Android Studio中内置的布局检查器。

Today i have upgraded from Android Studio 3.0 to 3.1 but I can´t find the Android Device Monitor!

How can access to the Android Device Monitor in Android Studio 3.1?

Per the Android Studio 3.1 release notes:

Device Monitor available from the command line

In Android Studio 3.1, the Device Monitor serves less of a role than it previously did. In many cases, the functionality available through the Device Monitor is now provided by new and improved tools.

See the Device Monitor documentation for instructions for invoking the Device Monitor from the command line and for details of the tools available through the Device Monitor.

So you need to manually start it from the command line if you still want to run the Device Monitor. Otherwise, migrate to the updated performance tools and updated Layout Inspector built directly into Android Studio.