2 Categories of Metrics for measuring App Performance that Everyone needs to know

User Acquisition Metrics

Total downloads per day/ month/ year
Downloads by country
Downloads by device type
Installations of app compared to installations of all apps in the same app category
Ratio of users installing app vs uninstalling app is an indicator of the success of user acquisition process at the moment
Spike in daily installs is often related to app's usability and stability
All installation metrics are collected at the server side within the marketplace
To collect installation numbers, it is not necessary to instrument a mobile app

Measured at runtime directly through all running app instances worldwide
Active users are defined as users who at least start the application in a given time frame
An active user is only counted once, even if the user starts several sessions within the given time period
DAU and MAU are common metrics
These users have the potential to become loyal, revenue-generating customers

Defined as one use of the app by a user which begins when user starts the app and ends when the app is suspended to the background.
The end of a session is defined by the tracking tool. Most app-monitoring frameworks stop a session when the app is suspended to the background for a specific amount of time (Eg- 30 seconds in Google Analytics)
The number of sessions per day shows how often users open the app
The session length measures median session length over time, instead of the average to reduce impact of outliers.
The session length and number of sessions per user depend on the type of app. Eg- A messaging app is characterized by very short sessions but a high number of user sessions per day while a music streaming app may be started once and keep playing in the background for long

Retention rate measures the chances that a user installing the app will still use the app after N days
Common ways to measure retention rate are measuring fixed retention for Day 1, Day 7 and Day 30 or rolling retention.
Rolling retention = Number of users returning to the app on Day N / Number of users who installed the app on Day 0. N is typically 1/3/7/30/90 days.
A heat map is a popular way of showing retention over several weeks. It shows a matrix of values - rows with different Day 0 dates and columns (Day 1, Day 2 etc) highlighting number of users who opened the app after Day 0
Full retention is also sometimes though it is quite restrictive in the sense it measures users who come back every single day until Day N
Classic or fixed retention measures all users who come back exactly on Day N