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According to Google's Material Design Guidelines, would it be considered appropriate to have a filter functionality triggered by a floating action button?
The guidelines suggest only to use a floating action button for the most prominent actions on the page, so I'm wondering if this kind of an action would justify utilizing a floating action button.
If the FAB is only to filter, I would ask myself what is the best location for the filter button for that button to be in context with the information that it is filtering? Filter buttons are commonly placed above the information that it is filtering. Where do you plan to place the filter button?
Secondly, when information is scrolling below a floating filter button does the button remain in context?
You mention 'the most prominent actions' but it should generally be singular. If there are several equally valid options such as picking an image from a gallery, a FAB shouldn't be used (guidelines mention this).
When you read between the lines, an important part is that the FAB is independent.
When you open gmail and calendar, both have a FAB that allows you to create a new item. But if you are in an item, there is no FAB. Similarly if you are in Maps, it shows a navigation button, but when you start typing it disappears and a similar-functioned button 'directions' button appears. In all three cases, the action is decoupled from any particular event/mail/location.
A filter is not technically decoupled from the items, but it's coupled equally to each item, so you could argue that's the same rule through a different lens.
Even more abstractly, the button has the single function of "create new [generic item]". Which includes "create new filter... but not modifying the filter. Which means that you should remove the button once you've made the initial filter. Or that every time you press the button you have to start create an entirely new one which means having to enter things like min-max price all over again. Neither solution is good for usability.
So taking a strict view, a filter is ambiguous in terms of neutrality and gives issues down the line.
In practicality though? They're guidelines, not rules. And people will intuitively understand that the button simply means 'filter' instead of 'make/edit'. So you can probably get away with it just fine - although it depends on the context and complexity of the overall interface.
