Firebase Analytics custom events params

[Update, May 2017]

As of May 2017, custom parameter reporting is now supported in Google Analytics for Firebase. Please refer to this help center article for more details.

your custom data and parameters will be available as soon as your audience reach 10 or more, that is a privacy restriction. so just use it in your activity as:

FirebaseAnalytics mFirebaseAnalytics = FirebaseAnalytics.getInstance(this);
Bundle bundle = new Bundle();
bundle.putString("some_key", "some_value");
mFirebaseAnalytics.logEvent("some_name", bundle);

it will work (after some time (max 24 hrs) you can see some_name as event in your event view but some_key will be available when audience is 10 or more).

As of https://support.google.com/firebase/answer/7397304?hl=en&ref_topic=6317489, you need to register your parameters before they can be shown

When you first set up custom parameters, a data card for it will be added to your event detail report. However, it may take up to 24 hours for any data to appear.