Is your feature request related to a problem? Please describe.
The Android SDK provides a way to set information (email, id, name, additional properties) via the setUserInfo method. The way our application is architected, we have additional properties that we may want to set throughout the lifecycle of the application. For example, you could consider a contrived example to be if a user were to purchase a premium subscription in an application, we may want to tag them with the is_premium_user extra attribute.

Describe the solution you'd like
It would be nice to be able to just be able to set additional attributes without having to additionally set the name / email / id again.

So when I first have access to name, email, id, I make a call to:

                    Datadog.setUserInfo(
                        userId,
                        name,
                       email
                    )

Later, after the user has done something we'd like to mark their account with an extra property, it'd be nice to see a method like:

                    Datadog.addUserProperty(
                        key,
                        value,
                    )

that would act similarly as if I had passed in an extraProperties map when initially called setUserInfo

Describe alternatives you've considered
I tried calling the original setUserInfo with null name / email / id hoping it would respect the previously set values, but it does not appear to be the case.

Additional context
Add any other context or screenshots about the feature request here.