A user on the XDA Developers forum stumbled across Google Assistant integration in Android Messages, and the seemingly limited feature test manifests in a very similar manner to the Google Assistant features that can be found in Google's Allo chat app for Android. Namely, the Assistant-based smart replies that present relevant suggestions like weather and travel time, then use Google Assistant to fulfill those suggestions have been showing up in Android Messages for some people. The rollout seems fairly random at the moment, and not many users have reported it.
Those who do happen to get selected for the testing can expect to see Google Assistant interjecting on their conversations by offering up helpful suggestions. When tapped on, these suggestions launch Assistant into action, retrieving and serving info in mid-conversation. Available suggestions thus far seem to limited to more mundane requests, such as weather information and restaurant recommendations. Since the feature was meant to learn and grow over time on Allo, the same will likely happen here. This means that the feature will make its way out to more users in time, and as it's used more often and gets to see more conversations unfold, it will get better at figuring out what people want at any given time and how best to deliver on those ideas and requests.
Google Allo never really got off the ground; Google built a good service around the app, but a number of key features missing at launch meant that it didn't see the kind of early adoption numbers necessary to give the service a long and popular life. Allo is still available, but user numbers are fairly low, and many of its key features can only be used in chats with other Allo users. Android Messages, on the other hand, gets its advanced messaging chops from the Rich Communication Services protocol, which was co-created by Google and is fast becoming a standard part of modern Android handsets. Between a web client and now what seems to be a test of Google Assistant integration, Android Messages is starting to look more and more like Google's preferred platform for injecting its expertise into mobile chat services.