Problems

1. magento_indexer_backlog_count_total metric calculates backlog items in a wrong way
   
   
   I expect it to display the same backlog items count as bin/magento indexer:status does.

2. magento_indexer_backlog_count_total metric always has 3 possible values (per each indexer status)

But it doesn't give information what is the current status. So it's impossible to really understand how many items are in the backlog

Solution

1. For the first problem I applied the same solution as Magento used for bin/magento indexer:status command. See \Magento\Indexer\Console\Command\IndexerStatusCommand::execute. It uses direct SQL requests, so it has the same performance as before fix applying
2. I removed 'status' and 'isValid' labels, so now we have only the title of the index and its backlog items count. To be able to see the current status we need to have different metric.

Note

I have added data patch that removes magento_indexer_backlog_count_total metrics from db table to make sure that only data with the new structure will be collected