[Telemetry] MyRocks usage tracking

Problem: We don't have a number of  "How many MyRocks engines used"

We can use "mysql_global_variables_rocksdb_block_cache_size" variable to identify "server with myRocks engine"

Let's improve our telemetry with a number of monitored MySQL servers with Myrocks  based on these metrics (VM)

For presentation, it's ok just have a total number (same as we have for MySQL)