How to implement Firebase into a microservice architecture

I already have an existing microservice app.

Now I want to add a simple database to simply grab contact information during events, this information will be edited once with a boolean to define if a user has been contacted yet or not.

I was thinking of making a table in a relational database to keep track of this but I think a NoSQL database would be better for this. So I decided to use Firebase, now my question is:

Where would this fit in my architecture?

[Front end] -> [API Gateway] -> [Service 1, Service 2, etc]

I was thinking of 2 options:

1. Putting it directly into the front end (as far as I know this is the intended Firebase use)
2. Putting it in the API Gateway

Both solutions are valid. In my case I decided to run with the second approach, passing trough the API to keep true to my Single source of truth