Hey there. I have a running instance of NR and the bridge to GH made by @i8beef which is working great. I wonder if there is a way to ask to GH for values that can be accessed via MQTT from my sensors such as "Hey Google, what's the humidity in the flowerpot?".
That depends upon the bridge, can you provide a link to whatever you are using.
It depends on what device you are using. Google Home's grammar is both more and less flexible than Alexa in this regard, i.e., it supports more queries and terminology, but is still fairly rigid in reality to what it'll understand, and Google STILL doesn't document what these utterances are very well.
So that disclaimer aside, yes, anything that's reported to Google as state for the system is a candidate for such queries if Google supports it. I.e., "is the kitchen light on" will answer because Google supports that voice query.
Arbitrary sensor types aren't supported though, they have to be things supported by Google, which is still pretty anemic. Someone on (https://github.com/actions-on-google/smart-home-nodejs/issues/253) was able to accumulate a list of currently undocumented traits / device types that haven't been released / finalized yet, but we don't have complete information on them. I've implemented a couple as beta (Volume and Channel for instance, because I wanted to play with them personally). There is a HUMIDIFIER device type in that list, as well as "Sensor" and "SensorState" traits that looks like it'd come in soon, but until we have complete documentation I can't really implement.