Prometheus

Prometheus (source code) is an open source monitoring tool that can be used to instrument and report on Python web applications.

This primer on Prometheus walks through installation, configuration and metrics collection.
Monitoring synchronous Python web apps with Prometheus and its asynchronous monitoring counterpart are two tutorials that show how to add middleware to your web apps that allows Prometheus metrics collection.
Monitoring with Prometheus provides an overview of the tool and explains how to combine it with Grafana to visualize the metrics that are collected.
Monitor your applications with Prometheus is a getting started guide with a walkthrough of how to instrument a simple Golang application.
Custom Application Metrics with Django, Prometheus, and Kubernetes shows how to handle the initial configuration with django-prometheus, deploys the Django web app using Helm and configures Prometheus to scrape metrics from the application running on Kubernetes.
A gentle introduction to the wonderful world of metrics has a quick summary that compares Prometheus with Nagios, then digs into the logging format and what you can visualize with this tool.