When thousands of Client connections are established against ProxySQL, the performance of Active Frontend Connections leads to a graph that doesn't render. Further this can make the rendering and general usage of ProxySQL Instance Summary dashboard feel very sluggish.
We should find a way to use topk or other limiter in order to reduce the impact of analyzing +10k metric series, specifically series proxysql_processlist_client_connection_list
This is expected to be a somewhat common occurrence as one of the value propositions of ProxySQL is the connection pooling feature, so thousands of client connections is expected.

Steps to Reproduce

1. Launch PMM Server against ProxySQL v2.4 + PXC or Percona Server for MySQL
2. Launch 10,000 client connections against ProxySQL
3. Load ProxySQL Instance Summary dashboard