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Zabbix
Real-time monitoring of IT components and services, such as networks, servers, VMs, applications and the cloud.
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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
Setup Zabbix (https://www.zabbix.com/) and use SNMP (search for a template for your switches, chances are someone has created one) to pull throughput data from the switchport your proxies are connected to. This will graph them for you on a continual basis, you can then setup some triggers (alerts) that will flag over-utilization (say >80Mbps) for you, can generate an email or SMS alert based on that or just see it in the dashboard.