Is there any api for dashboard analytics data?
From the "dashboard.concurrentUsers" in your question, it looks like you're trying to get the data from the Analytics tab of your dashboard.
As of Oct/Nov 2020 there is an Analytics Data API that you can use to run Analytics reports and retrieve data
You can also enable the BigQuery integration. Doing this means that all Google Analytics for Firebase events from that moment on will be written to BigQuery, where you can query them. This is the raw data, the events written by your apps, and not the aggregated data that you find in the dashboard.